Branched off #101 so merge that first.

Implements literals in the spec with a ~ prefix so that for example with "~alpha": ["egg", "tadpole", "frog"] the parameter value will be passed through as a list rather than being expanded into multiple leaves. The parameter name is converted to alpha i.e. loses the prefix. Also works with values that are dicts and that appear as macros, generators, evaluators or equations e.g. "~alpha": "$NotAMacro" will pass through "$NotAMacro" as parameter value.

Literals can also be specified value-side as a string such as:

• "alpha": "~[1, 2, 3]"
• "alpha": "~$NotAMacro"

If a literal is specified on both the key and value, then the value is taken as literal (allowing a parameter value to start with ~) such that "~alpha": "~I like tildes" evaluates to a parameter of name alpha with value "~I like tildes".