add string lower and string upper

[krader1961]

I was working on a proof of concept implementation to support zsh style tied variables to address issue #436. But I found myself needing a way to lowercase a string. Now that we have a string builtin it shouldn't be necessary to run an external command like tr to do this operation.

[faho]

This doesn't do full unicode locale-aware uppercasing:

$ string toupper "aßb"
AßB

(which should be "ASSB" if the locale is de_DE.UTF-8)

This is okay (this functionality is unlikely to be used and brings with it a bunch of issues), but should probably be documented.

[zanchey]

See also #2241.

[krader1961]

This doesn't do full unicode locale-aware uppercasing.

Correct. But that's true of all the non-ASCII handling within fish. The only feasible way to fix it is to use the ICU or Boost libraries.

[zanchey]

git revert 0d64bbed46f07f78a24449139d11e7fafe968e4e? 😂

[floam]

Am I crazy or should it be possible with the STL in C++?

https://stackoverflow.com/a/17993266