[4.0] Re-worked parser to support escaping dollars

[GrahamCampbell]

Changes:

1. The parser now returns a special Value object instead of a pure string. The value object contains both the parsed string, and the locations of nested environment variables for interpolation.
2. Interpolation of environment variables is now performed right to left instead of left to right. This allows one to write code such as "${${FOO}}" where FOO holds the name of the actual variable.
3. Double quoted strings now support escaping dollars with a backlash.
4. Escape sequences are now banned in single quoted strings, just like for unquoted strings. This includes disallowing escaped (or otherwise) single quote characters within a single quoted string.

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Closes #378.