Add three functions that trim characters from the front & end of a
`StringRef`. All functions return a new `StringRef` that references a
sub-string of the original `StringRef`.
- `trim(chars_to_remove)`: strips all characters from the start and end
that occur in `chars_to_remove`.
- `trim(char_to_remove)`: same, but with a single character to remove.
- `trim()`: remove leading & trailing whitespace, so same as
`trim(" \r\n\t")`
Reviewed By: JacquesLucke
Differential Revision: https://developer.blender.org/D12031
7e91a60be6