[ES6 modules] Introduce ModulatorImpl

This CL introduces ModulatorImpl, which is an implementation of
"settings object" spec concept + serves as a backplane for tieing
all ES6 module algorithm components together.

See below diagram for where it stands in the stack:
https://docs.google.com/document/d/1vjiWxwhg9D0GNNOYgw3AxMG0iKOC9I3jlID4GTgZsac/edit#heading=h.47x0qrpzjbj4

BUG=594639

Review-Url: https://codereview.chromium.org/2826803004
Cr-Commit-Position: refs/heads/master@{#465506}
4 files changed