Segwitv0 compilation of x-only keys panics

[shesek]

It should Err, but currently panics. The PR adds a test case demonstrating the issue (but not a fix):

test policy::compiler::tests::segwitv0_xonly_pk ... FAILED

---- policy::compiler::tests::segwitv0_xonly_pk stdout ----
thread 'policy::compiler::tests::segwitv0_xonly_pk' panicked at src/policy/compiler.rs:526:52:
Terminal creation must always succeed: ContextError(XOnlyKeysNotAllowed("08c0fcf8895f4361b4fc77afe2ad53b0bd27dcebfd863421b2b246dc283d4103", "Segwitv0"))

Parsing a wsh(pk(XONLY)) straight into a Descriptor does work as expected and raises an Err, as covered by this test case.

[apoelstra]

Good find! I suspect I broke this in #732.