Skip convertToR search for values with are SymbolicExpressions

Currently, RInteropInternal.convertToR searches through its various conversion methods even when passed a value which is already a SymbolicExpression (or a subclass of SymbolicExpression). It seems like it might be a useful optimization to check for SymbolicExpressions first, and skip the search if one is found.

[tpetricek]

This sounds like a good idea! If you can submit a pull request, that would be great!

PS: Sorry for a slow ping, I'm finishing a new release and finally going through all the old issues....

[AndrewIOM]

Implemened in #271 so that any values already in R are passed through first

    let toR (eng: RInterop.RInstance) (value: obj) : SymbolicExpression =
        match value with

        // Pass-through of basic R expression values:
        | :? SymbolicExpression as s -> s
        | :? RProvider.Abstractions.RExpr as s -> RExprWrapper.toRBridge s
        | :? RBridge.Extensions.REnvironment as s -> s.AsSymbolicExpression