Fix several differences in the parsing of $.. and ${...}

Fix several differences in the parsing of $.. and ${...}

Namely:
    * The first character in ${...} used to have no restrictions
    * ${foo:bar} used to be legal
    * ${foo::bar} worked, but ${foo'bar} didn't

And possibly other subtle, so far undiscovered bugs. This was
resolved by simply using the same code for both things.

Note that this commit is not entirely useful on its own; While
tests pass, it requires changes from the following commit to work
entirely.