Tracking issue: flatten the eval stack

[sorpaas]

Currently, if core finds an opcode it cannot handle, it will Trap first, and then Runtime will attempt to eval it. This is rather inefficient.

Instead, core should expose the eval table (which is just 256-item fn pointers), to allow Runtime to customize it. Machine would now take a generic parameter stateS (with default ()), and then pass it as the final parameter to eval fns. Runtime then should fully build with that, and only trap for CALL/CREATE. Gasometer should be the only thing that wraps a Machine.

This should make it more predictable when reasoning about performance, while not losing much abstractions.