std.ArrayHashMap: add reinit method and other housekeeping, including the move towards "unmanaged" containers

[andrewrk]

std.ArrayHashMap is now deprecated and aliased to std.ArrayHashMapWithAllocator.

To upgrade, switch to ArrayHashMapUnmanaged which will entail updating callsites to pass an allocator to methods that need one. After Zig 0.14.0 is released, std.ArrayHashMapWithAllocator will be removed and std.ArrayHashMapUnmanaged will be a deprecated alias of ArrayHashMap. After Zig 0.15.0 is released, the deprecated alias ArrayHashMapUnmanaged will be removed.

This move comes from unanimous agreement among veteran Zig users who have converged on the "unmanaged" container variants. They act as better building blocks, avoiding storing the same data redundantly, and the presence/absence of the allocator parameter dovetails nicely with the reserve capacity / reserved insertion pattern.

[mlugg]

I know ArrayList is a little more dicey since we want to rename it, but this PR could also do HashMap, and any others I'm forgetting about.

[linusg]

What's the rationale for the last commit? My understanding is that "gpa" stands for "general purpose allocator", a specific implementation that can provide the allocator interface. Similarly "arena" is a specific allocator implementation. Using these to refer to arbitrary allocator instances seems unintuitive.

[leroycep]

My understanding of the gpa convention is that it more refers to a class of implementations; that is allocators which are designed to general case where you need allocation. arena means an allocator that doesn't require memory to be individually freed.

Now in general developers will just use the allocators provided by the standard library, but that is not required. The gpa and arena conventions are shorthands telling API consumers how the allocator will be used in this specific API.