Implements avoid loading `rlang` where not required

[shikokuchuo]

As per the suggestion by Winston Chang.

• Implements required weakref functions for internal package use only.
• conditions rlang::as_function() usage only where !is.function()

In addition:

• uses native symbol registration - this is a real speedup, apart from just being 'recommended practice'.
• removes blanket Rcpp import, just the selective import is fine

Closes #186.

[wch]

This is great. Can you also add an entry to the NEWS.md file?

I'm curious: how much of a speed improvement does the symbol registration provide?

There could be another boost from removing the Rcpp dependency and replace it with cpp11, but that would be a more difficult project.

[shikokuchuo]

Have added the comments and a NEWS item.

The saving from native symbol registration is every time there is a .Call(). As dynamic lookup involved a string search... I haven't benchmarked but remember it being slow. I contributed this change to the torch mlverse package some time ago, and as it is so atomic there (nearly every operation is an underlying tensor operation hence a .Call), the impact was not negligible.

[wch]

Thanks!