Planning module: Missing varint length in size calculation of some placeholders

[jp1ac4]

As mentioned in #481 (comment), it appears that the planning module doesn't include the varint length for TapScript and TapControlBlock when getting the size, but does for other placeholders:

rust-miniscript/src/util.rs

Lines 36 to 37 in acbd120

	Placeholder::TapScript(s) => s.len(),
	Placeholder::TapControlBlock(cb) => cb.serialize().len(),

Should the varint length be included here for consistency with other placeholders?

Please also see #701 for some other inconsistencies.

[apoelstra]

Yeah, I think the varints should always be included since they're part of the witness cost.