single subprocess per TERM type

[haliphax]

Right now, a new subprocess is spun up for each connection in order to isolate a curses terminal with the connection's term type. Instead, a dict of subprocesses with termtype as the key could be used such that only a single subprocess per term type would be necessary. This should double the number of connections that the system is able to maintain (roughly, in theory).

Original idea credit to jquast, as with most of the terminal package.

[haliphax]

inkey will be hard af with this.

[haliphax]

may be superseded by #111

[haliphax]

need to test rich and textual to see if they respect the _environ setting or if it is per-process