Add `br(map_reader)` attribute

[jam1garner]

Motivation

Being able to utilize reader adapters easily would greatly improve multiple areas of binrw that could use a bit of improvement:

• Parsing from decompressed or encrypted content
• Ensuring a field doesn't read past a specific size
• Easier "parse until end of section" ability (something akin to the .take(...) io::Read adapter[^1])

[^1]: io::Take does not support seeking so this functionality would also be needed (likely 2 variants, one for "seeks relative to current position" and one for "seeks relative to start of inner reader")

Syntax

#[br(map_reader = $map_fn:expr)]

[csnover]

I feel like this, or something very similar, would also be the correct solution for issues like #102.

Also, regarding footnote 1, I already have a TakeSeek type that can be used.