Procs and funcs that are compiled into the Oils binary.
Naming convention that allows us to compare OSH code size versus bash:
builtin/
func_hay.py # functions only appear in YSH
method.py # methods for all types, could be split up further
io_osh.py # echo builtin
# read builtin is in OSH, with YSH enhancements
io_ysh.py # write builtin is only in YSH
pure_osh.py # set, shopt
pure_ysh.py # append
trap_osh.py # trap
json_ysh.py # json