Move crates/api to crates/wasmtime (#1693)

The `wasmtime` crate currently lives in `crates/api` for historical
reasons, because we once called it `wasmtime-api` crate. This creates a
stumbling block for new contributors.

As discussed on Zulip, rename the directory to `crates/wasmtime`.

scripts/publish-wasmtime.sh

@@ -32,7 +32,7 @@ for cargo_toml in \
     crates/profiling/Cargo.toml \
     crates/debug/Cargo.toml \
     crates/jit/Cargo.toml \
-    crates/api/Cargo.toml \
+    crates/wasmtime/Cargo.toml \
     crates/wasi/Cargo.toml \
     crates/wast/Cargo.toml \
     crates/misc/rust/macro/Cargo.toml \