Start using macro-paradise compiler plugin.

This commit introduces a 2.10.2-compatible version of Macro Paradise,
which lets us use quasiquotes to construct trees. This mostly replaces
the need for manual tree construction and destructuring, although
there may still be some cases where working with trees directly is
clearer.

This commit fixes the most confusing and egregious instances of manual
tree building. There is still room for more improvement here.