Optimize `Allocator::grow` and `Allocator::shrink` for the last allocation in `ngx_pool_t`.

[bavshin-f5]

Does ngx_pool_t do non-noop frees if done in LIFO order? If so, then it would also make sense for it to support in-place reallocs for the last allocation, and override the provided default methods for Allocator::grow and Allocator::shrink here as well.

Allowing the last allocation to grow in-place, even in bump allocators, is particularly nice when building Vecs of an unknown number of elements and which end up resizing and growing their underlying storage.

Originally posted by @fitzgen in #164 (comment)

The approach has been used in ngx_array_push since nginx 0.1, and the underlying structures are quite stable. The optimization would be quite beneficial for building arrays and strings on pool.