Try initializing namespaces lazily

[jcburley]

This is something of a proof of concept, as it doesn't seem to speed up official Joker much that I can measure on the two machines I'm using while on vacation (and my main development machine, at home, is even less likely to show much of a difference).

But, applied to my gostd fork, the resulting Joker definitely starts up faster, though still not as quickly as official Joker. (I think there's room for further improvement, in terms of delaying var-time initialization until Init() is called. That might help things further.)

That suggests this PR, or something like it, could keep Joker startup times from getting much longer even if/as more namespaces and/or functions (vars) are added to its default library.

Note that, while trying different ideas out, I was disappointed to discover that Go currently does not offer much in the way of practical build-time initialization, beyond scalar constants. That is, arrays and maps, in particular, cannot be initialized at build time and thus be present, already-initialized, when the resulting executable starts up. I hope to see improvements in this situation someday soon, and might look into how feasible it is to do and possibly offer to help with them.

[candid82]

This is a pretty neat trick. Thanks!