Auto merge of #124987 - workingjubilee:macro-metavar-expr-with-a-shorter-len, r=c410-f3r,joshtriplett,joshtriplett

Rename `${length()}` to `${len()}`

Implements the rename suggested in rust-lang/rust#122808 (comment)
> I brought this up in the doc PR but it belongs here – `length` should probably be renamed `len` before stabilization. The latter is de facto standard in the standard library, whereas the former is only used in a single unstable API. These metafunctions aren’t library items of course, but should presumably still be consistent with established names.

r? `@c410-f3r`

Author: bors