additional lexer tokens in unit test

Should increase lexer coverage to roughly 100%.
fixed PoolAllocator assertion issue

The PoolAllocator would refuse to allocate fixed size chunks of memory, because
an assertion checked whether the memory requirement for the aligned version was
*less* than the allocated worst-case size, rather than *less-or-equal*.
added partial keywords to lexer test

Added partial keywords to the lexer.vhd test file in order to cover the many
`return 0;` statements in keywords.hpp. Should increase coverage to almost
100%.