Adding snapshot forwarding.

[orizi]

Removing not yet audited libfunc nullable_match_snapshot using this feature.

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This change is 

[gilbens-starkware]

Reviewed 11 of 11 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @orizi)

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corelib/src/box.cairo line 21 at r1 (raw file):

        unbox(self)
    }
    fn as_snapshot(self: @Box<T>) -> Box<@T> nopanic {

Suggestion:

#[inline(always)]
fn as_snapshot(self: @Box<T>) -> Box<@T> nopanic {

[gilbens-starkware]

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @orizi)