Logging - The `wasm-bindgen` Guide

# Logging

The `wasm_bindgen_webidl` crate (used by `web-sys`'s `build.rs`) uses `env_logger` for logging, which can be enabled by setting the `RUST_LOG=wasm_bindgen_webidl` environment variable while building the `web-sys` crate.

Make sure to enable "very verbose" output during `cargo build` to see these logs within `web-sys`'s build script output.

    cd crates/web-sys
    RUST_LOG=wasm_bindgen_webidl cargo build -vv

If `wasm_bindgen_webidl` encounters WebIDL constructs that it doesn't know how to translate into `wasm-bindgen` AST items, it will emit warn-level logs.

    WARN 2018-07-06T18:21:49Z: wasm_bindgen_webidl: Unsupported WebIDL interface: ...