Provide the C WASI implementation as an option.

This adds the C WASI implementation as a new crate, wasmtime-wasi-c,
and adds a command-line flag to the wasmtime command-line driver to
select which WASI implementation to use.

wasmtime-wasi-c/sandboxed-system-primitives/CMakeLists.txt

@@ -0,0 +1,8 @@
+cmake_minimum_required(VERSION 3.0)
+project(SandboxedSystemPrimitives C)
+
+include_directories(include)
+
+add_library(SandboxedSystemPrimitives STATIC src/posix.c src/random.c src/str.c)
+
+install(TARGETS SandboxedSystemPrimitives DESTINATION .)