Encode a custom "producers" section in wasm files

Encode a custom "producers" section in wasm files

This commit implements WebAssembly/tool-conventions#65 for wasm files
produced by the Rust compiler. This adds a bit of metadata to wasm
modules to indicate that the file's language includes Rust and the
file's "processed-by" tools includes rustc.

The thinking with this section is to eventually have telemetry in
browsers tracking all this.