Marked unsafe regions

[ss2165]

Guppy allows a few unsafe operations. The most notable is linear array indexing which may fail at runtime (e.g. using the same index twice, bounds overflow).

While we can encourage practices which reduce the use of this (e.g. list comprehensions, constant folding to check statically known indices etc.) it seems unlikely we can eliminate entirely.

One option to make this easier to manage is follow Rust's unsafe example and have explicitly annotated regions

@guppy.unsafe
def indexed_cx(qs: array[qubit, N], i: int, j: int):
    cx(qs[i], qs[j])

Calls to unsafe functions would only be allowed in other unsafe functions.