No way for webppl consumer to access semFuncs

[JsonFreeman]

The semFuncs collection is useful for a consumer to write their own grammar. There are two use cases that are not supported.

1. A grammar author accessing existing functions in semFuncs. This is just a matter of exporting semFuncs from the library.
2. Allowing users to augment semFuncs. This is in theory possible, but a new function written in webppl will not be called correctly by the parser. I don't see a way around this. It's an instance of a more general problem with webppl/js compatibility.