Update API documentation.
Update import. #performance
Add the Hoa\Compiler\Exception\Lexer exception.
mv matchesLexem matchLexeme
Prevent a token that matches an empty value.

If a token matches an empty value, a Hoa\Compiler\Exception\Lexer will
be thrown.
Optimize lexing. #performance

We replace a call to strpos() with a modified regular expression. In
fact, “0 === strpos(…)” is strictly equivalent to “^($regex)” in the
preg_match() call.
Moreover, we can remove a call to count().