Container types

[danakj]

• HashMap
• HashSet
• BTreeMap
• BTreeSet
• VecDeque
• ? FlatMap and FlatSet
• ? Bucket Array, or some kind of Vector that has pointer stability
• InlineVector (all storage is inline, so no allocs, but same API as Vec, just can't go past the fixed capacity)
• Vec (still some methods left to fill out) Flesh out the Vec and Slice APIs and implementations #142
• Slice and SliceMut (still one method to do) Slice::select_nth_unstable_by #233
• ? Opaque Opaque<T> that can be converted to/from Vec<T> #145
• String and str (or StringSlice) without UTF8 requirements Strings and Chars #144
• ? UTF-8 String and str Strings and Chars #144
• ? OSString and OSStr, probably not cuz the base String isn't UTF-8 Strings and Chars #144
• Allocator API to use in allocating containers Allocator API for individual container objects to use #278
• Replaceable Global allocator used when no allocator is specified Allow providing a custom global allocator #143
• Cow Cow type (Clone-on-write) #404

Got a ? on the ones I'm not sure should be part of Subspace.