Move the C API to a separate crate (#818)

* Move the C API to a separate crate

This commit moves the C API from `crates/api/src/wasm.rs` to
`crates/capi/src/lib.rs` to be located in a separate crate. There's a
number of reasons for this:

* When a Rust program depends on the `wasmtime` crate, there's no need
  to compile in the C API.
* This should improve compile times of the `wasmtime` crate since it's
  not producing artifacts which aren't always used.
* The development of the C API can be guaranteed to only use the public
  API of the `wasmtime` crate itself.

Some CI pieces are tweaked and this overall shouldn't have much impact
on users, it's intended that it's a cleanup/speedup for developers!

* Disable rustdoc/tests for capi

* Review feedback

* Add back in accidentally deleted comment

* More renamings

* Try to fix dotnet build

crates/c-api/examples/Makefile

@@ -0,0 +1,155 @@
+###############################################################################
+# Configuration
+
+# Inherited from wasm-c-api/Makefile to just run C examples
+
+WASM_FLAGS = -DWASM_API_DEBUG  # -DWASM_API_DEBUG_LOG
+C_FLAGS = ${WASM_FLAGS} -Wall -Werror -ggdb -O -fsanitize=address
+CC_FLAGS = -std=c++11 ${C_FLAGS}
+LD_FLAGS = -fsanitize-memory-track-origins -fsanitize-memory-use-after-dtor
+
+C_COMP = clang
+
+WASMTIME_API_MODE = debug
+
+
+# Base directories
+WASMTIME_API_DIR = ..
+WASM_DIR = wasm-c-api
+EXAMPLE_DIR = ${WASM_DIR}/example
+OUT_DIR = ${WASM_DIR}/out
+
+# Example config
+EXAMPLE_OUT = ${OUT_DIR}/example
+EXAMPLES = \
+  hello \
+  callback \
+  trap \
+  start \
+  reflect \
+  global \
+  table \
+  memory \
+  hostref \
+  finalize \
+  serialize \
+  threads \
+  # multi \
+
+# Wasm config
+WASM_INCLUDE = ${WASM_DIR}/include
+WASM_SRC = ${WASM_DIR}/src
+WASM_OUT = ${OUT_DIR}
+WASM_C_LIBS = wasm-bin wasm-rust-api
+WASM_CC_LIBS = $(error unsupported C++)
+
+
+# Compiler config
+ifeq (${WASMTIME_API_MODE},release)
+  CARGO_BUILD_FLAGS = --release
+else
+  CARGO_BUILD_FLAGS =
+endif
+
+ifeq (${C_COMP},clang)
+  CC_COMP = clang++
+  LD_GROUP_START =
+  LD_GROUP_END =
+else ifeq (${C_COMP},gcc)
+  CC_COMP = g++
+  LD_GROUP_START = -Wl,--start-group
+  LD_GROUP_END = -Wl,--end-group
+else
+  $(error C_COMP set to unknown compiler, must be clang or gcc)
+endif
+
+WASMTIME_BIN_DIR = ${WASMTIME_API_DIR}/../../target/${WASMTIME_API_MODE}
+WASMTIME_C_LIB_DIR = ${WASMTIME_BIN_DIR}
+WASMTIME_C_LIBS = wasmtime
+WASMTIME_CC_LIBS = $(error unsupported c++)
+
+WASMTIME_C_BINS = ${WASMTIME_C_LIBS:%=${WASMTIME_C_LIB_DIR}/lib%.a}
+
+###############################################################################
+# Examples
+#
+# To build Wasm APIs and run all examples:
+#   make all
+#
+# To run only C examples:
+#   make c
+#
+# To run only C++ examples:
+#   make cc
+#
+# To run individual C example (e.g. hello):
+#   make run-hello-c
+#
+# To run individual C++ example (e.g. hello):
+#   make run-hello-cc
+#
+
+.PHONY: all cc c
+all: cc c
+cc: ${EXAMPLES:%=run-%-cc}
+c: ${EXAMPLES:%=run-%-c}
+
+# Running a C / C++ example
+run-%-c: ${EXAMPLE_OUT}/%-c ${EXAMPLE_OUT}/%.wasm
+	@echo ==== C ${@:run-%-c=%} ====; \
+	cd ${EXAMPLE_OUT}; ./${@:run-%=%}
+	@echo ==== Done ====
+
+run-%-cc: ${EXAMPLE_OUT}/%-cc ${EXAMPLE_OUT}/%.wasm
+	@echo ==== C++ ${@:run-%-cc=%} ====; \
+	cd ${EXAMPLE_OUT}; ./${@:run-%=%}
+	@echo ==== Done ====
+
+# Compiling C / C++ example
+${EXAMPLE_OUT}/%-c.o: ${EXAMPLE_DIR}/%.c ${WASM_INCLUDE}/wasm.h
+	mkdir -p ${EXAMPLE_OUT}
+	${C_COMP} -c ${C_FLAGS} -I. -I${WASM_INCLUDE} $< -o $@
+
+${EXAMPLE_OUT}/%-cc.o: ${EXAMPLE_DIR}/%.cc ${WASM_INCLUDE}/wasm.hh
+	mkdir -p ${EXAMPLE_OUT}
+	${CC_COMP} -c ${CC_FLAGS} -I. -I${WASM_INCLUDE} $< -o $@
+
+# Linking C / C++ example
+.PRECIOUS: ${EXAMPLES:%=${EXAMPLE_OUT}/%-c}
+${EXAMPLE_OUT}/%-c: ${EXAMPLE_OUT}/%-c.o ${WASMTIME_C_BINS}
+	${CC_COMP} ${CC_FLAGS} ${LD_FLAGS} $< -o $@ \
+		${LD_GROUP_START} \
+		${WASMTIME_C_BINS} \
+		${LD_GROUP_END} \
+		-ldl -pthread
+
+# Installing Wasm binaries
+.PRECIOUS: ${EXAMPLES:%=${EXAMPLE_OUT}/%.wasm}
+${EXAMPLE_OUT}/%.wasm: ${EXAMPLE_DIR}/%.wasm
+	cp $< $@
+
+###############################################################################
+# Wasm C / C++ API
+#
+# To build both C / C++ APIs:
+#   make wasm
+
+.PHONY: wasm wasm-c wasm-cc
+wasm: wasm-c wasm-cc
+wasm-c: ${WASMTIME_C_BIN}
+wasm-cc: ${WASMTIME_CC_BIN}
+
+${WASMTIME_C_BINS}: CARGO_RUN
+	cd ${WASMTIME_API_DIR}; cargo build --lib ${CARGO_BUILD_FLAGS}
+
+.PHONY: CARGO_RUN
+CARGO_RUN:
+
+
+###############################################################################
+# Clean-up
+
+.PHONY: clean
+clean:
+	rm -rf ${OUT_DIR}
+