Move br(count) attribute to desugar to binrw::helpers::count

[jam1garner]

Unlike the count attribute, which is limited to supporting Vec<T>, the count helper function works for any type which implements FromIterator<T>.

Upsides:

• Less magic implementation, more type safe
• Adds support for more types

Possible pitfalls:

• Perf regression due to the lack of "fake specialization" provided for Vec<u8>
• Breaking some existing use-case maybe?

[roblabla]

FWIW, if there's a measurable perf regression, this sounds exactly like the kind of use-cases where autoref specialization could be used to recover the lost perf.