[Chore] Remove `Box`ing of `SymbolicExpression`s in Clarity

[cylewitruk]

SymbolicExpressions are unnecessarily Box'd in a few places in the clarity crate when used in Vecs. As Vecs are already on the heap, a Box is not necessary.

[blockstack-devops]

This issue has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.