[spec/interpreter/test] Spec & implement proposal

[rossberg]

This should include everything necessary for stage 3:

• Specify syntax of annotations.
• Implement annotation parsing in interpreter.
• Test for annotation parsing.
• Bonus: specify @name and @custom annotations in spec appendix.

Like with the custom sections, the interpreter only implements generic parsing of annotation syntax and otherwise ignores it.

[rossberg]

@lukewagner, friendly ping.