Attributes

[harpocrates]

Attributes are currently sometimes not parsed, not printed, and not tested. The following needs to happen:

• go through the parser to find where attributes ought to be parsed. To do this, see which nodes on the AST support attributes and then figure out where those attributes fit in concrete syntax.
• add test case files in sample-sources specifically for this as part of rustc-tests (since attributes are constantly evolving).
• fix Diff to support attributes
• modify the rustc-tests to also check that pretty-print and re-parse is a no-op; this will check that attributes are properly pretty-printed.
• update the AST form of attributes (relevant PR)