Implemented a more space efficient string<->integer map. (#9113)

Summary:

While investigating memory consumption, I noticed that the tiktoken loader was allocating 16MB of memory, mainly distributed over two std::unordered_maps.  These two maps (the Encode and Decoder types) in Tiktoken are inverses of each other, and the std::string objects contained therein are clones of each other.

The allocation of a node in each map is 40 bytes (on aarch64 Android):
* 2x doubly linked list pointers at 8 bytes each
* 1 std::uint64_t (8 bytes)
* 1 std::string (12 bytes, std::strings contain an internal buffer for small strings).

Each node actually allocates 48 bytes of usable memory, as the allocator aligns the allocations to 16 byte boundaries.

This implementation of the string/integer map has several features:
* Sharing of the data payload between two hash indices.
* Variable sized integers, variable sized string length fields and best fit allocation of string data.  That is to say, the data payload elements are variable sized.

The implemented unit tests tracks the memory size allocated between the old std::unordered_map method and the new StringIntegerMap method, yielding a ~6x improvement in the memory allocated:
```
string integer map size = 2623343
unordered map size = 16078928
```

There was a significant speedup when looking up strings, although looking up integers was about the same:

```------------------------------------------------------------------------------------------------
Benchmark                                                      Time             CPU   Iterations
------------------------------------------------------------------------------------------------
BM_FindStringIntegerMapString/iterations:100                4722 us         4722 us          100
BM_FindStringIntegerMapInteger/iterations:100                529 us          529 us          100
BM_FindStringIntegerMapStringOptional/iterations:100        4714 us         4713 us          100
BM_FindStringIntegerMapIntegerOptional/iterations:100        537 us          536 us          100
BM_FindStdUnorderedMapString/iterations:100                 7128 us         7127 us          100
BM_FindStdUnorderedMapInteger/iterations:100                 536 us          536 us          100
```

Reviewed By: swolchok, larryliu0820

Differential Revision: D69472841

Author: Daniel Deptford