[RDY] improve compile time tests coverage

[jangko]

The goal of this PR is to have 100% coverage for all exported API to pass compile time test.
The compile time helpers implementation might be not efficient, but first make it run then make it run fast.
As a bonus, it also improve runtime coverage.
zero, negative numbers, hi and lo tests also added if possible.
Inputs are welcome.

• uint bitops2
• uint endians2
• io
• int addsub
• int bitwise
• int comparison
• int muldiv
• uint addsub
• uint bitwise
• uint comparison
• uint exp
• uint modular arithmetic
• uint muldiv

[jangko]

this PR is ready for review.
Summary:

• there are no new features except truncate now able to truncate to full range of native ints.
• all exported API are covered by compile time test. fixed Improve compile time test coverage #89 and probably Improve test coverage #67.
• fixed various bugs encountered.
• implement missing compile time functionalities.
• pass all tests with Nim 0.19.6 and 1.0.2. while Nim 0.20.0 and 1.0.0 posses critical compile time bugs.

[mratsim]

Looks Great to me :)

[arnetheduck]

@kdeme this looks like an interesting fuzzing target, wouldn't you say?