* Convert JOB_* enums to scoped enums
* Convert standalone job_is_* functions to member functions
* Convert standalone job_{promote, signal, continue} to member functions
* Convert standolen job_get{,_from_pid} to `job_t` static functions
* Reduce usage of JOB_* enums outside of proc.cpp by using new
`job_t::is_foo()` const helper methods instead.
This patch is only a refactor and should not change any functionality or
behavior (both observed and unobserved).
Internally fish should store vars as a vector of elements. The current
flat string representation is a holdover from when the code was written
in C.
Fixes#4200
It's bugged me forever that the scope is the second arg to `env_get()`
but not `env_set()`. And since I'll be introducing some helper functions
that wrap `env_set()` now is a good time to change the order of its
arguments.
