functions: add helpers to convert case

Posix shell can do a lot of things, but there is no native way to change
the case of a string. Some packages require this functionality, and they
use `tr` in a subshell to achieve that.

In the global scope of APKBUILDs we want to avoid exec in the global
scope. By providing helper functions, we can avoid this.

The functions only handle the basic ascii letters, which should suffice
for the usecases that are needed in APKBUILD files.