Use v0.13.0 of swift-parsing.
No functionality changes.
Additional documentation and unit tests

Added factorial function.

Revert to older ignoreSpaces parser since it works on older compilers.

Reintroduce identifier splitting when implied multiplication is active.

Expand API with parseResult and evalResult calls to obtain additional information about the failure cases.

Modifications to work with swift-parsing v0.12. No API change.

Fix for new warning from Xcode.