Tweak naming and ordering in `StringReader::bump()`.
Tweak naming and ordering in `StringReader::bump()`.

This patch removes the "old"/"new" names in favour of "foo"/"next_foo",
which matches the field names.

It also moves the setting of `self.{ch,pos,next_pos}` in the common case
to the end, so that the meaning of "foo"/"next_foo" is consistent until
the end.

aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust