Merge wasi-common into wasmtime

This commit merges [CraneStation/wasi-common] repo as a subdir of
this repo while preserving **all** of git history. There is an
initiative to pull `wasi-common` into [CraneStation/wasmtime], and
[CraneStation/wasmtime] becoming a monorepo. This came about for
several reasons with a common theme of convenience, namely,
having a monorepo:
1. cleans up the problem of dependencies (as we have seen first
   hand with dependabot enabled, it can cause some grief)
2. completely removes the problem of syncing the closely dependent
   repos (e.g., updating `wasi-common` with say a bugfix generally
   implies creating a "sync" commit for pulling in the changes into
   the "parent" repo, in this case, `wasmtime`)
3. mainly for the two reasons above, makes publishing to crates.io
   easier
4. hopefully streamlines the process of getting the community
   involved in contributing to `wasi-common` as now everything
   is one place

[CraneStation/wasi-common]: https://github.com/CraneStation/wasi-common
[CraneStation/wasmtime]: https://github.com/CraneStation/wasmtime

wasi-common/Cargo.toml

@@ -0,0 +1,55 @@
+[package]
+name = "wasi-common"
+version = "0.5.0"
+authors = ["The Wasmtime Project Developers"]
+edition = "2018"
+license = "Apache-2.0 WITH LLVM-exception"
+description = "WASI implementation in Rust"
+categories = ["wasm"]
+keywords = ["webassembly", "wasm"]
+repository = "https://github.com/CraneStation/wasmtime"
+readme = "README.md"
+
+[features]
+# this feature requires wasm32-wasi target installed, and it enables wasm32
+# integration tests when run with `cargo test --features wasm_tests`
+wasm_tests = []
+
+[dependencies]
+wasi-common-cbindgen = { path = "wasi-common-cbindgen", version = "0.5.0" }
+failure = "0.1"
+libc = "0.2"
+rand = "0.7"
+cfg-if = "0.1.9"
+log = "0.4"
+filetime = "0.2.7"
+lazy_static = "1.4.0"
+num = { version = "0.2.0", default-features = false }
+wig = { path = "wig", version = "0.1.0" }
+
+[target.'cfg(unix)'.dependencies]
+nix = "0.15"
+
+[target.'cfg(windows)'.dependencies]
+winx = { path = "winx", version = "0.5.0" }
+winapi = "0.3"
+cpu-time = "1.0"
+
+[dev-dependencies]
+wasmtime-runtime = { path = "../wasmtime-runtime" }
+wasmtime-environ = { path = "../wasmtime-environ" }
+wasmtime-jit = { path = "../wasmtime-jit" }
+wasmtime-wasi = { path = "../wasmtime-wasi" }
+wasmtime-api = { path = "../wasmtime-api" }
+cranelift-codegen = "0.49"
+target-lexicon = "0.8.1"
+pretty_env_logger = "0.3.0"
+tempfile = "3.1.0"
+
+[build-dependencies]
+cfg-if = "0.1.9"
+
+[lib]
+name = "wasi_common"
+crate-type = ["rlib", "staticlib", "cdylib"]
+