Allocator interface: make shrinkFn optional

[andrewrk]

I propose that shrinkFn be an optional pointer. An implementation of the Allocator interface would set this to null when it does not provide a way to reclaim memory. The Allocator interface wrapper code would provide a shim implementation of shrink which simply did return old_mem[0..new_size];.

The benefit of this proposal is that the Allocator interface could have a new function called, perhaps, canReclaimMemory() which is simply self.shrinkFn != null. If canReclaimMemory returns false then the user of the allocator knows they don't have to bother with free().

This would be used, for example, here:

zig/std/zig/parse.zig

Lines 12 to 13 in bddbbef

	pub fn parse(allocator: *mem.Allocator, source: []const u8) !ast.Tree {
	var tree_arena = std.heap.ArenaAllocator.init(allocator);

This code currently unconditionally creates an arena allocator (which does not reclaim memory with free()) on top of the provided allocator. This is a waste of memory and computations if the provided allocator already does not reclaim memory. So it would be adjusted to only create the arena allocator if the provided allocator reported true for canReclaimMemory.

Finally, this would also simplify some allocator implementations since they could just do .shrinkFn = null rather than provide a useless shrink function.

[andrewrk]

I've got this solved in the null-shrink-fn branch, but it's blocking on #2377.

[andrewrk]

Taking this in the direction of #4431