Allow manipulating strings directly?

[lgarron]

This is less performant, but super convenient:

> expand("(R U)2")
"R U R U"

Can easily be done by separating the Traversal[Down]Up.traverse() entry point from a recursive entry point, and translating the input from/to a string if it was given as a string.

The main problem I see is that the performance issue might be masked, and users might perform a lot of conversions back and forth:

structureEquals(invert(expand("(R U)2")), expand(invert("(R U)2")))

[lgarron]

Other options:

Include separate versions of the functions for strings:

expandString("(R U)2")

Create a wrapper function that converts to/from strings:

forStrings(expand)("(R U)2")