feat: add result.Map() helper

chore: update linter config to resolve all warnings

fix: report subscriber location when ack panics

eg.
```
~/…/types/foo $ go run .                                                                                                                                                                                     master
panic: ack timeout for /Users/aat/dev/types/foo/main.go:12: main.main

goroutine 3 [running]:
github.com/alecthomas/types/pubsub.(*Topic[...]).run(0x10010b8a0)
	/Users/aat/dev/types/pubsub/pubsub.go:206 +0x42c
created by github.com/alecthomas/types/pubsub.New[...] in goroutine 1
	/Users/aat/dev/types/pubsub/pubsub.go:71 +0x10c
exit status 2
```

fix: give the pubsub channel 16384 elements of capacity

feat: add Either

feat: add must and once packages

feat: add From() functions to Result and Option

feat(pubsub): support nacking

Nacking a message will return an error to any synchronous publishers.

fix: ensure async subscriptions are closed

This bug slipped in in the previous change to support synchronous
publishers/subscribers because there was no test for it. Test added.

feat: add support for synchronous pubsub

This allows publish actions to block until all subscribers have
acknowledged receipt of the message.