Fix warnings for signedness

[axic]

Part of #311.

[chfast]

This goes in wrong direction. Unsigned types should be avoided. They are only good for bitfields and bit manipulation.

[chfast]

Do not hide a cast.

[axic]

Unsigned types should be avoided. They are only good for bitfields and bit manipulation.

I don't get it, why?

[chfast]

They are only good for bitfields and bit manipulation.

[axic]

Still, why?

[chfast]

I had to look it up. Most of the arguments are listed here (read all section): https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#arithmetic.

I should add that you can detect signed overflow with Undefined Behavior Sanitizer (with the newest version you can also detect unsigned overflows, but because they are valid in C++, you can get false positives). Together with fuzz testing this gives incredible results.

[codecov-io]

Codecov Report

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The diff coverage is 91.07%.

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[axic]

As discussed the specification isn't clear what the types of these are, but implicitly it feels they are defined as unsigned in the EEI. Added an issue to review that: ewasm/design#132

Merging this change as it doesn't changes semantics.