Use try_from instead of the cast crate.

`try_from` is newly stable in the standard library starting in Rust 1.34.

Cargo.toml

@@ -11,7 +11,6 @@ license = "Apache-2.0 WITH LLVM-exception"
 
 [dependencies]
 wasi-common-cbindgen = { path = "wasi-common-cbindgen" }
-cast = "0.2"
 failure = "0.1"
 libc = "0.2"
 rand = "0.6"
@@ -27,4 +26,4 @@ winapi = { version = "0.3", features=["std", "handleapi", "processthreadsapi", "
 name = "wasi_common"
 crate-type = ["rlib", "staticlib", "cdylib"]
 
-[workspace]
+[workspace]