replace linear cap search with trie.

replace linear cap search with trie.

for every input, termbox used to iterate over termcaps one
at a time, `strncmp`ing it to the input buffer. the benefit
of this approach was that it had a very simple
implementation. the downside was that it was inefficient,
increasingly so as we added more caps such as modified
arrow keys.

the trie makes it easy to detect when there are overlaps in
termcaps. the obvious one is `TB_KEY_ESC` versus all other
escape codes. in the future we can modify function
`extract_esc_cap` to, e.g., potentially return `TB_KEY_ESC`
in `TB_INPUT_ALT` mode if no further input is available
after a certain amount of time.