Moved `lexer.h` to `lexer/lexer.h`

(for the planned rewrite of `reaver::lexer` using `reaver::parser`).
Kdevelop files update
Use C++14.
Update TODO
Little further work on `reaver::format`.
Fix parsers following moving of `lexer.h`.
`elf::i386`
`literal` parser.
Add elf test.
Add traits for `literal` parser.
`thread_pool`: fix name qualification, add `abort()`

Also starting adding "automatic" callbacks for `thread_pool`. This will
be used for example by thread pools used as "callbacks" for events; some
example will one day appear in README.