Using `StackFallbackAllocator.get()` multiple times is a footgun.

[IntegratedQuantum]

Zig Version

0.11.0-dev.3910+689f3163a

Steps to Reproduce and Observed Behavior

Calling StackFallbackAllocator.get() (re-)initializes the internal FixedBufferAllocator.
This means that calling StackFallbackAllocator.get() multiple times causes problematic behavior:

const std = @import("std");

pub fn main() !void {
	var allocator = std.heap.stackFallback(4096, std.heap.page_allocator);
	const a = try allocator.get().alloc(u8, 1);
	const b = try allocator.get().alloc(u8, 1);
	std.debug.assert(a.ptr != b.ptr);
}

This causes an assertion failure, meaning that both allocations returned the same pointer.
In actual code this is hard to debug because it leads to weird behavior.
For example in my code it messed up line wrapping and swallowed characters in text rendering.

Expected Behavior

The other allocators allow calling .allocator() multiple times without causing any trouble.
I think this should also apply to StackFallbackAllocator.get().

I think the internal FixedBufferAllocator should be initialized only once, preferably in the heap.stackFallback(..) function.

Note: This would be a breaking change, that's why I'm hesitant to implement this myself. Existing code might be relying on StackFallbackAllocator.get() to initialize the FixedBufferAllocator