Rust: Slightly refactor/comment the number literal scanning.
Rust: Slightly refactor/comment the number literal scanning.

One change introduced by this is that the malformed literal is detected a
little earlier in some cases than before.

rhaberkorn/scintilla-mirror,rhaberkorn/scintilla-mirror,rhaberkorn/scintilla-mirror,rhaberkorn/scintilla-mirror,rhaberkorn/scintilla-mirror,rhaberkorn/scintilla-mirror,rhaberkorn/scintilla-mirror,rhaberkorn/scintilla-mirror