[swift-evolution] [Returned for revision] SE-0077: Improved operator declarations

[Anton Zhilin]

I replaced `precedencegroup` with `precedence` and added `Precedence` 
suffix to all precedence group names. See:

https://github.com/Anton3/swift-evolution/blob/fix-operator-
precedence/proposals/0077-operator-precedence.md

My feelings:
1. `precedencegroup` describes what it declares more precisely
2. `precedence` is shorter (partially compensating for longer names)
3. `precedence` can be correctly interpreted as "precedence level"
4. `precedence` looks nicer overall

5. `Precedence` suffix is bulky. One must specify it in operator 
declarations and in all relationships
6. All groups ending with adjectives may unambiguously drop the suffix
7. Number of such groups is small. New groups will tend to be named after 
corresponding operators, and they will be noun-based

Questions:
1. Where else can we drop -Precedence?
2. Can we make more names end with adjectives?