Method to remove values from Punctuated

[dtolnay]

The only current APIs on syn::punctuated::Punctuated that result in the Punctuated becoming smaller are pop (remove last) and clear (remove all).

https://github.com/dtolnay/syn/issues/343#issuecomment-657923539 mentioned needing to remove by index.

[Caellian]

drain_filter removes Clone requirement for processed data and halves number of loops for use cases such as specializing generic arguments. 🥺