[release-branch.go1] go/ast: ast.Print must not crash with unexported fields

[release-branch.go1] go/ast: ast.Print must not crash with unexported fields

««« backport d134e30c4d29
go/ast: ast.Print must not crash with unexported fields

Don't print unexported struct fields; their values are
not accessible via reflection.

Fixes #3898.

Also:
- added support for arrays
- print empty maps, arrays, slices, structs on one line
  for a denser output
- added respective test cases

R=r
CC=golang-dev
https://golang.org/cl/6454089

»»»