Fix closure specialization issue

[MilesCranmer]

Fixes #5 and other issues related to the use of closures preventing specialization.

This solves the issue by generating two functions:

1. with a new name, but the old body
2. with the old name, but a type-checking body

Thus the specialization will be as good as normal Julia code.

[coveralls]

coverage: 98.571% (-0.5%) from 99.107% when pulling 89498e47b504e9089cd4c1ca63ab3fc28d6e5d98 on fix-closures into 99ff00dbad672b854bf5805c6e3a19e5ad40e5cb on main.