Loading multiple dynamic libraries with the same bindings

[dcharkes]

See dart-lang/sdk#42082.

While I think the use case is a bit uncommon, we should explore enabling this. However, preferably without complicating the API for the case in which there is just one.

Currently we have a top-level init, and top-level functions that can be called.

DynamicLibrary _dylib;

/// Initialises dynamic library
void init(DynamicLibrary dylib) {
  _dylib = dylib;
}

int sum(int a,int b) {
  return _sum(a,b);
}

final _dart_sum _sum = _dylib.lookupFunction<_c_sum, _dart_sum>('sum');

typedef _c_sum = Int32 Function(Int32 a,Int32 b);

typedef _dart_sum = int Function(int a,int b);

We could consider wrapping those in a class that has as a single member the DynamicLibrary and having all the functions as methods on the object.

That does slightly regress the usage, but if people use it with named imports, it does not really matter whether they write named_import.someFfiFunction() or an_object.someFfiFunction(), and they can just make that object a toplevel definition that they import everywhere.

@mannprerak2, maybe we should address this before releasing v0.1 because it changes the API.