Rollup merge of #40129 - abonander:proc_macro_bang, r=jseyfried

Implement function-like procedural macros ( `#[proc_macro]`)

Adds the `#[proc_macro]` attribute, which expects bare functions of the kind `fn(TokenStream) -> TokenStream`, which can be invoked like `my_macro!()`.

cc rust-lang/rfcs#1913, #38356

r? @jseyfried
cc @nrc

diff --cc src/libproc_macro/lib.rs
Simple merge

diff --cc src/libsyntax/feature_gate.rs
Simple merge