Fix jashkenas#3597: Allow interpolations in object keys

The following is now allowed:

    o =
      a: 1
      b: 2
      "#{'c'}": 3
      "#{'d'}": 4
      e: 5
      "#{'f'}": 6
      g: 7

It compiles to:

    o = (
      obj = {
        a: 1,
        b: 2
      },
      obj["" + 'c'] = 3,
      obj["" + 'd'] = 4,
      obj.e = 5,
      obj["" + 'f'] = 6,
      obj.g = 7,
      obj
    );

- Closes jashkenas#3039. Empty interpolations in object keys are now _supposed_ to be
  allowed.
- Closes jashkenas#1131. No need to improve error messages for attempted key
  interpolation anymore.
- Implementing this required fixing the following bug:

      ("" + a): 1

  The above used to error out on the colon, saying "unexpected colon". But
  really, it is the attempted object key that is unexpected. Now the error is on
  the opening parenthesis instead.
- However, the above fix broke some error message tests for regexes. The easiest
  way to fix this was to make a seemingly unrelated change: The error messages
  for unexpected identifiers, numbers, strings and regexes now say for example
  'unexpected string' instead of 'unexpected """some #{really long} string"""'.
  In other words, the tag _name_ is used instead of the tag _value_.
  This was way easier to implement, and is more helpful to the user. Using the
  tag value is good for operators, reserved words and the like, but not for
  tokens which can contain any text. For example, 'unexpected identifier' is
  better than 'unexpected expected' (if a variable called 'expected' was used
  erraneously).
- While writing tests for the above point I found a few minor bugs with string
  locations which have been fixed.