Added implementation for i128 mul.

[orizi]

Stack:

• Added implementation for i128 mul. #3625 ⬅
• Added cast for signed integers. #3623

⚠️ Part of a stack created by spr. Do not merge manually using the UI - doing so may have unexpected results.

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This change is 

[gilbens-starkware]

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

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corelib/src/test/integer_test.cairo line 1745 at r1 (raw file):

    assert_eq(@(-3_i128 - -1_i128), @-2_i128, '-3 - -1 == -2');
    assert_eq(@(-231_i128 - -131_i128), @-100_i128, '-231--131=-100');
    assert_eq(@(1_i128 * 3_i128), @3_i128, '1 * 3 == 3');

Consider adding a 1*0 test case.

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corelib/src/test/integer_test.cairo line 1748 at r1 (raw file):

    assert_eq(@(2_i128 * 4_i128), @8_i128, '2 * 4 == 8');
    assert_eq(@(-1_i128 * 3_i128), @-3_i128, '-1 * 3 == 3');
    assert_eq(@(-2_i128 * 4_i128), @-8_i128, '-2 * 4 == 8');

Suggestion:

    assert_eq(@(-1_i128 * 3_i128), @-3_i128, '-1 * 3 == -3');
    assert_eq(@(-2_i128 * 4_i128), @-8_i128, '-2 * 4 == -8');

[gilbens-starkware]

Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @orizi and @spapinistarkware)

[gilbens-starkware]

Reviewed 2 of 2 files at r2, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @orizi and @spapinistarkware)

[orizi]

Reviewed 1 of 2 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @spapinistarkware)