asm parser lacking `label` parse functionality

In rust's asm.rs I misunderstood eat_keyword(label) for "tracks if there is any number that is followed by a :" to be a label. The former is explicitly a label and the latter is a local label.

The former needs to be parsed explicitly and put in a hashmap or set to be expanded later.

The latter can be implicitly parsed via parse_format_strings, it is passed straight into the assembler.

For docs
rust zulip asm discussion
arm's docs on local label
rust's asm goto
rust's tracking issue

Start in Rust's asm.rs, then read the linked Zulip discussion, ARM local-label documentation, and Rust asm-goto tracking issue. Confirm the distinction between explicit labels and local labels: explicit labels should be retained for later expansion, while local labels continue through parse_format_strings. Done means both forms are parsed according to that distinction.