Improve generated code for derived instances

[lukaszcz]

• Closes Improve the inlining optimization to handle derived instances well #3186
• Changes the generated code for derived Eq instances to define a local recursive function with a let instead of a lambda. This is necessary, because otherwise an indirection is generated on each recursive call -- instance resolution inserts the instance identifier, which then is matched on and the function for the recursive call extracted instead of being called directly. This cannot be (easily) optimized in Core because, to avoid looping in the compiler, in general one cannot do inlining of recursive functions.
• Minor improvements to the inlining optimization and the main optimization phase: do inlining before specialization and more liberal rules for heuristic case value inlining.