Operators `__static_assert` and `__static_check`

[Daniel-Cortez]

What this PR does / why we need it:

Implements operators __static_assert and __static_check using @YashasSamaga's implementation as a base (see #300).

The most notable differences from the original implementation are:

• do_static_check() reuses the standard string parsing mechanism for the optional 2'nd argument, in the same manner as operator __pragma does.
• Both __static_check and __static_assert return a bool:-tagged result.
• The use of parentheses is now mandatory. This is because otherwise this kind of use:

  __static_assert 1, __static_assert 2;

  would be ambiguous, as there will be no way to tell if it should be treated as __static_assert(1), __static_assert(2); or __static_assert(1, __static_assert(2));.

Which issue(s) this PR fixes:

Fixes #294

What kind of pull this is:

• [ ] A Bug Fix
• [x] A New Feature
• [ ] Some repository meta (documentation, etc)
• [ ] Other

Additional Documentation:

This PR is built upon the changes from #602, so you'll want to review and merge that PR first.

[Daniel-Cortez]

Rebased to the latest changes from #602.

[Daniel-Cortez]

Implemented a new mechanism that allows to record tokens read by function lex(). With this, __static_assert and __static_check can now properly display multi-line expressions in error/warning messages.