treemap: add a find_mut method
hashmap: add find_mut method
trie: add find_mut method
smallintmap: add find_mut method
expose find_mut in the Map trait
trie: improve docstring
auto merge of #5528 : thestinger/rust/find_mut, r=brson

This currently requires workarounds for the borrow checker not being flow-sensitive for `LinearMap` and `TrieMap`, but it can already be expressed for `TreeMap` and `SmallIntMap` without that.