rt: low-level runtime support

The rt (runtime) module provides low-level, non-portable access to the host, and
support code for the Hare compiler and standard library. The use of this module
is not recommended for most user programs, and any program which uses rt is
unlikely to be portable. However, any program which needs to make syscalls
directly will have to use rt to do so.

Documentation for most interfaces is not provided; refer to the host
documentation (e.g. Linux man pages) instead.
