slice: Improve operator overloads (cover more cases).
Few fixes after new slice operator overloads.
Add implicit C array -> slice conversion constructor.
string: Implement operator+.
string: More operator+ specifications, cover all cases.
Implement zbs::unicode::utf8 module.
Add missing license notice to test/test_unicode_utf8.cc.
Add missing unicode::utf8 tests.
Rename tests: test_*.cc -> *.cc.
Mirror header directories structure in the source dir.
Make doxygen optional for `waf configure`.
Add unicode::utf8 docs.
Constexpr slice mania.
Fix slices test. Use slices::sort instead of std::sort.
Hash implementation for strings, will be used for future map container.
Use debug flags always for now.
Rename _utils.cc -> utils.cc.

Only header files should follow that convention.
Add a bunch of utilities.

That includes a fastrand PRNG which is useful for certain purposes and a
malloc/free which does abort() on failure.
Use internal malloc/free for all the raw allocation purposes.
Small documentation fix.

It was introduced after moving from ordinary function to constexpr ones.