if-let in net, plugin, script: #4153

Old PR: #4128
Due to extensive problems encountered in merging with upstream changes, I decided to work on a new branch instead. This commit follows instructions given by @Manishearth in Critic for the old PR.

Replaced appropriate simple `match` statements involving destructure with `if let` and those not involving destructure with `if`. Those with matches to `atom!`s are left unchanged.
Crates affected:
- net
- plugin
- script