Correctly parse class name in 'for my class $foo

The code in toke.c on encountering 'for my' or for our',
skips past any optional package name, then checks that the next thing is a
'$', so that it can give a "Missing $ on loop variable" error if
necessary.

However, the code to skip the package name was using scan_ident() rather
than scan_word(); the latter assumes that the first char was the sigil,
and starts scanning from the second char onwards. So in something like

    for my a1b $x (...)

scan_ident() starts scanning at '1b' in 'a1b', thinks it scanning $1 or
similar, and stops at the first non-digit char, in this case the b.

The fix is to use parse_word() instead.
add comment to foreach toking code

to make it clear what its doing.