rm FIXME from LinearMap's to_bucket

There are no more poor hash functions left in the codebase, and it makes
sense to rely on there being a good hash function thanks to the
inclusion of SipHash and the ease of using it with custom types.

Closes #3041
add reserve_at_least method to LinearMap/LinearSet
auto merge of #4959 : thestinger/rust/map, r=graydon