Move `UntrustedString` and `PrintableString` to `lightning-types`

Move `UntrustedString` and `PrintableString` to `lightning-types`

`lightning-invoice` currently has a dependency on the entire
`lightning` crate just because it wants to use some of the useful
types from it. This is obviously backwards and leads to some
awkwardness like the BOLT 11 invoice signing API in the `lightning`
crate taking a `[u5]` rather than a `Bolt11Invoice`.

This takes one more step, moving the `UntrustedString` and
`PrintableString` types to `lightning-types`.