This eliminates the "missing" notion of env_var_t. Instead
env_get returns a maybe_t<env_var_t>, which forces callers to
handle the possibility that the variable is missing.
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
This is the first step to implementing issue #4200 is to stop subclassing
env_var_t from wcstring. Not too surprisingly doing this identified
several places that were incorrectly treating env_var_t and wcstring as
interchangeable types. I'm not talking about those places that passed
an env_var_t instance to a function that takes a wcstring. I'm talking
about doing things like assigning the former to the latter type, relying
on the implicit conversion, and thus losing information.
We also rename `env_get_string()` to `env_get()` for symmetry with
`env_set()` and to make it clear the function does not return a string.
