Add C/JS APIs to copy expressions

[dcodeIO]

This API enables use cases where we want to keep the original expression, yet utilize passes like vacuum or precompute to evaluate it without implicitly modifying the original.

C-API: BinaryenExpressionCopy(expr, module)
JS-API: Module#copyExpression(expr)

[dcodeIO]

The test currently fails because I made it to, due to the following problem: When copying an expression, it is straight-forward to make the copied outer expression known to API tracing, but not so straight-forward to do the same for its sub-expressions (see the expressions[0]). My intuition would be that we need a tracing-aware copy algorithm here, but that also means duplicating most of ExpressionManipulator::copy. Any suggestions?

[dcodeIO]

A radical solution might be to remove tracing entirely, since it complicates / limits the C-API with very little benefit imho. Even though it seems nice to have on paper, I actually never used it. Also caught myself several times worrying about the subtle slowdown it introduces since there are like a quadrillion C-API calls in the AssemblyScript compiler.

[kripken]

I'm open to removing the tracing API. It was very useful originally with a few early embedders of the C API, but I can't remember the last time it was helpful. Maybe we are stable enough it's not that necessary.

[dcodeIO]

Going to give that a try in a separate PR :)

[dcodeIO]

With API tracing removed this should be good now as well :)

[aheejin]

Sorry for the delay!