Plot compile to lambdify compile

[mmatera]

@bdlucas1, this is a proposal about where I think the lambdify code should be located. If this is not too disruptive, the next thing would be to use this code in the different places where we need compiled versions of WL expressions, like in table generations, numerical integration, and optimizers.

[bdlucas1]

If this is not too disruptive,
From that perspective I'm fine with this, if you think it's mature enough to be useful.

use this code in the different places where we need compiled versions of WL expressions
I have no comment on this because I'm not familiar with those uses, other than to say I'm not sure how complete function coverage is for lambdify_compile - not really sure how exhaustive the list of tests is, and of those a lot fail, although those mostly tend to be obscure numerical functions.

One minor comment on the import in plot3d_vectorized.py which I will add, otherwise lgtm.

[bdlucas1]

A minor point - I think it would be better to import as lambdify_compile, and change the one use of it in this file, to get rid of the name plot_compile altogether.