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/src/README.md

@@ -0,0 +1,14 @@
+This directory consists of selected files copied from the [libemulator]
+directory in the [cloudabi-utils] repository, with minor modifications,
+along with the accompanying LICENSE file from that repository.
+
+The modifications are marked with `WASMTIME_*` preprocessor macros.
+
+The files were copied at git revision
+be1ce21e1dded9c0c0a6ebe144cbea01cf44a874
+which is dated
+Sun Jan 13 23:26:03 2019 +0100
+.
+
+[libemulator]: https://github.com/NuxiNL/cloudabi-utils/tree/master/src/libemulator
+[cloudabi-utils]: https://github.com/NuxiNL/cloudabi-utils