Recently we added segwit v0 script checks to the `Address::from_script`
method, following the principle that we want to provide some safety for
users in the 'main' rust-bitcoin API.

At the time we did not add taproot checks because doing so broke one of
the test vector unit tests. Turns out the unit test was doing
roundtripping of an address -> scipt_pubkey() -> address which is not
strictly required by BIP-350 (where the test vector comes from).

Modify the unit test by adding a boolean flag indicating whether the
address should roundtrip and only roundtrip test such addresses.

Add taproot script validity check to the `Address::from_script` method.

We recently decided to provide additional safety for users by not
creating addresses from certain invalid scripts.

Add rustdocs describing the return value and the safety guarantees we
are explicitly providing so as not to surprise users.

(See GitHub issue rust-bitcoin#955 for more context.)

Creating an address from an invalid taproot script should fail. Add a
unit test to verify it does.