Implement stringsSepBy1 parser

[rmunn]

This would fix #3.

I tried to write unit tests as well as implement the function, but I couldn't figure out how the existing stringsSepBy unit tests work in order to add a set of stringsSepBy1 tests (just like stringsSepBy but rejecting the case where the first parser doesn't match even a single time). Besides which, it wasn't immediately obvious to me how to get the code compiling on my Linux development box with VS Code + Ionide, so I couldn't have run the unit tests even if I had written them.

So I'm afraid you'll have to write the unit tests and make sure my code is correct before merging it in. But it's a very straightforward change, and I could copy the logic from manyStringsImpl just above. So I'm pretty sure that it's correct, I just couldn't prove it with unit tests.

[rmunn]

Note that in addition to documenting the stringsSepBy1 parser, I also fixed a small inconsistency in the stringsSepBy documentation.

[stephan-tolksdorf]

Thank you for your pull request. I will have time to work on this towards the end of the next week.

[stephan-tolksdorf]

Robin, the PR looks good! Could you add the stringsSepBy1 function to the fsi signature file, make stringsSepByImpl private and squash your commits? I will then merge the PR and add tests.

(I'm working with @neoeinstein on modernizing the VS solution and the build system, see #5, which will hopefully also make things easier for Ionide users.)

[rmunn]

Done. I also reversed my decision about changing the sp parser name to p in the stringsSepBy documentation: after adding stringsSepBy1 to the .fsi file, I noticed that you'd gone with a consistent naming scheme of using cp for char parsers and sp for string parsers. So instead, I fixed the places in the stringsSepBy documentation that were inconsistent with that scheme, and used sp for the docs of stringsSepBy1.

This should now be ready for you to merge and add the unit tests.