Variable interpolation in symbols

[MattWindsor91]

This PRQ changes the AST and internal representation of symbolic variables so that tokens of the form #<int> are parsed and stored as argument placeholders. This allows the Starling pretty printer, among other things, to interpolate arguments into those positions.

The pretty printer representation of a symbol %{foo #1 bar #2 baz}(x, y) has now changed to

(sym "foo x bar y baz")

This change will make it much easier immediately to work with symbolic proofs, but is mainly here to facilitate Grasshopper.

Summary of changes:

• Symbolic bodies are no longer strings but SymbolicSentences, lists of either string components or argument references.
• Symbolics are no longer Funcs but are a new type Symbolic<'Arg>. Sym<'R> is now a union of 'R and Symbolic<Expr<Sym<'R>>>.
• The pretty printer for model-level symbols now interpolates and returns the result as a string inside an S-expression. The AST pretty printer does not interpolate.
• Added a few tests.

There is currently no way to represent a # in a Starling symbol. If this becomes a problem we can change the parser to interpret ## as a #.

[MattWindsor91]

@BenSimner is this one you could check? If not then @septract is leading on Grasshopper so maybe this would be in scope for him to check

[septract]

This all looks good to me - @CaptainHayashi please go ahead and merge if you're happy.