feat: Default impls for signed integer types

[milancermak]

Signed integer types are missing Default. This PR adds it.

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This change is 

[orizi]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @milancermak)

[orizi]

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

a discussion (no related file):
@yuvalsw for 2nd eye.

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[milancermak]

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @orizi)

a discussion (no related file):
Bump.

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[orizi]

Reviewable status: all files reviewed, 1 unresolved discussion

a discussion (no related file):

Previously, orizi wrote…

@yuvalsw for 2nd eye.

@gilbens-starkware as well.

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[gilbens-starkware]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @orizi)