Better recognize invalid numbers

A number has to have at least one valid digit. prefixes and signs are optional and do not qualify for a number by itself.

git-svn-id: svn://svn.code.sf.net/p/amforth/code/trunk@1546 fd1c0b69-5d68-47f5-910c-86182ebb6ecd
Minor size optimizations

git-svn-id: svn://svn.code.sf.net/p/amforth/code/trunk@1547 fd1c0b69-5d68-47f5-910c-86182ebb6ecd