We forgot to decode (i.e. turn into nice wchar_t codepoints)
"byte_literal" escape sequences. This meant that e.g.
```fish
string match ö \Xc3\Xb6
math 5 \X2b 5
```
didn't work, but `math 5 \x2b 5` did, and would print the wonderful
error:
```
math: Error: Missing operator
'5 + 5'
^
```
So, instead, we decode eagerly.
* Try to set LC_CTYPE to something UTF-8 capable
When fish is started with LC_CTYPE=C (even just effectively, often via
LC_ALL=C!), it's basically broken. There's no way to handle non-ASCII
characters with a C locale unless we want to write our
locale-independent replacements for all of the system functions.
Since we're not going to do that, let's try to find *some locale* for
LC_CTYPE.
We already do that in __fish_setlocale, but that's
- a bit of a weird thing that reads unstandardized system
configuration files
- allows setting locale to C explicitly
So it's still easily possible to end up in a broken configuration.
Now, the issue with this is that there is (AFAICT) no portable way to
get a list of all allowed locales and C.UTF-8 is not standardized, so
we have no one locale to fall back on and are forced to try a few. The
list we have here is quite arbitrary, but it's a start.
Python does something similar and only tries C.UTF-8, C.utf8 and
"UTF-8".
Once C.UTF-8 is (hopefully) standardized, that will just start
working (tm).
Note that we do not *export* the fixed LC_CTYPE variable, so external
programs still have to deal with the C locale, but we have no real
business messing with the user's environment.
To turn it off: $fish_allow_singlebyte_locale, if set to something true (like "1"),
will re-run the locale initialization and skip the bit where we force
LC_CTYPE to be utf8-capable.
This is mainly used in our tests, but might also be useful if people
are trying to do something weird.
