```
$ ./tmp/testgo161718.sh ./buildtags/ ./printer/
+ go1.16.8 test ./buildtags/ ./printer/
ok github.com/mmcloughlin/avo/buildtags 0.001s
ok github.com/mmcloughlin/avo/printer 0.002s
+ go1.17.2 test ./buildtags/ ./printer/
ok github.com/mmcloughlin/avo/buildtags 0.001s
ok github.com/mmcloughlin/avo/printer 0.002s
+ gotip test ./buildtags/ ./printer/
ok github.com/mmcloughlin/avo/buildtags 0.001s
ok github.com/mmcloughlin/avo/printer 0.002s
```
Updates #183
Implements formatting of constraints according to the current Go version
supported syntax. This is achieved by delegating to go/format and extracting
out the resulting comments.
Also provides functions to query for constraint syntax support, which are
mainly intended for writing version-dependent tests.
Updates #183
