Remove numerous things deprecated during the 0.14 release cycle

[linusg]

Basically everything that has a direct replacement or no uses left.

Notable omissions:

• std.ArrayHashMap: Too much fallout, needs a separate cleanup.
• std.debug.runtime_safety: Too much fallout.
• std.heap.GeneralPurposeAllocator: Lots of references to it remain, not a simple find and replace as "debug allocator" is not equivalent to "general purpose allocator".
• std.io.Reader: Is being reworked at the moment.
• std.unicode.utf8Decode(): No replacement, needs a new API first.
• Manifest backwards compat options: Removal would break test data used by TestFetchBuilder.
• panic handler needs to be a namespace: Many tests still rely on it being a function, needs a separate cleanup.