feat: handle negative literal integers

[mablr]

Closes #560

• Updated LitKind::Number variant to handle negative numbers
• Folding of neg unop expr into negative lit - with LitKind::Number set to negative - at ast lowering stage

status

• Handle-1 as negative integer literal
• Handle by peeling -(1) or -((2)) as negative integer literal as well
• Handle-(-1) as unary op negate on negative integer literal (due to first item)
• Handle-(1 + 2) as unary op negate, bin op of two positive int literals

todo

• fix typeck handled by feat(sema): implicit integer conversions #564
• add tests (testcases from initial issue) may be introduced instead in the PR mentioned above as the implicit integer conversion would be nice to have a ui test...

[mablr]

This is not finished yet, but I've changed the target of my modifications from parser to sema's ast lowering (actually simpler than I thought).

Please let me know if I'm on the good way 😃

[mablr]

Fyi @onbjerg I tried to apply locally to this PR your patch #564 for implicit integer conversions.

So this test case passes successfully the checks 👍 :

echo 'contract C { function f() public { int8 a = -((1)); int8 b = -1; uint8 c = -(-1);  } }' | cargo run -- -Ztypeck -

but for the following I guess explicit conversion/evaluation is needed:

echo 'contract C { function f() public { uint8 a = -(1-2); } }' | cargo run -- -Ztypeck -