Parser: Ensure `parse_function` doesn't enter infinite loops

[gdelazzari]

Fixes #226 and other instances where the parser was getting stuck because an unexpected token was encountered, such as:

function foo<Ts..>(a: i32)

function foo<Ts, ..>(a: i32) 

function foo<Ts>(a: i32, ..)

[BuggieBot]

Hello!

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[AtkinsSJ]

Closed as this is a change for the now-gone Rust-based compiler. Feel free to port your changes to the new compiler, if they're not already added, and open a new PR. :^)