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.
The codegen crate contains the code to convert from MIR into LLVM IR,
and then from LLVM IR into machine code. In general it contains code
that runs towards the end of the compilation process.
For more information about how codegen works, see the rustc guide.
ab5e45ae6b