Disable rustdoc on the wasmtime cli binary (#834)

Currently the wasmtime binary (src/bin/wasmtime) and the wasmtime API
crate (crates/api) share the same output filename.  This causes the
output files of the wasmtime binary to clobber the output files of the
wasmtime API crate.   So running `cargo doc --all` will often not build
the wasmtime API docs, which is the more useful of the two.

This is a rustdoc bug, and can be worked around by disabling
documentation for the wasmtime binary.

Cargo.toml

@@ -16,6 +16,11 @@ publish = false
 [lib]
 doctest = false
 
+[[bin]]
+name = "wasmtime"
+path = "src/bin/wasmtime.rs"
+doc = false
+
 [dependencies]
 # Enable all supported architectures by default.
 wasmtime = { path = "crates/api" }