This was a workaround to be able to build on case-insensitive file
systems where it might get confused about <string.h> vs <String.h>.
Let's just not support building that way, so String.h can have an
objectively nicer name. :^)
Rewrite this function to go from left-to-right instead of right-to-left
since this allows us to accumulate valid characters before encountering
any invalid ones.
This fixes parsing of strings emitted by GCC, like "1, %0|%0, %1}". :^)
