reimplement std.HashMap

 * breaking changes to the API. Some of the weird decisions from before
   are changed to what would be more expected.
   - `get` returns `?V`, use `getEntry` for the old API.
   - `put` returns `!void`, use `fetchPut` for the old API.
 * HashMap now has a comptime parameter of whether to store hashes with
   entries. AutoHashMap has heuristics on whether to set this parameter.
   For example, for integers, it is false, since equality checking is
   cheap, but for strings, it is true, since equality checking is
   probably expensive.
 * The implementation has a separate array for entry_index /
   distance_from_start_index. Entries no longer has holes; it is an
   ArrayList, and iteration is simpler and more cache coherent.
   This is inspired by Python's new dictionaries.
 * HashMap is separated into an "unmanaged" and a "managed" API. The
   unmanaged API is where the actual implementation is; the managed API
   wraps it and provides a more convenient API, storing the allocator.
 * Memory usage: When there are less than or equal to 8 entries, HashMap
   now incurs only a single pointer-size integer as overhead, opposed to
   using an ArrayList.
 * Since the entries array is separate from the indexes array, the holes
   in the indexes array take up less room than the holes in the entries
   array otherwise would. However the entries array also allocates
   additional capacity for appending into the array.
 * HashMap now maintains insertion order. Deletion performs a "swap
   remove". It's now possible to modify the HashMap while iterating.

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