Refactor 0x.js exported interface

[fabioberger]

Description

Until now, we've been maintaining a legacy public interface for 0x.js. Now that we've broken up the package into several smaller packages that can be imported separately:

• @0xproject/contract-wrappers
• @0xproject/order-utils
• @0xproject/order-watcher

It makes sense to remove the legacy interface and instead, re-export the same exports from these individual packages as the exports available via 0x.js.

Benefits

• This will make it trivial for someone to migrate from 0x.js to only importing the sub-packages they actually use.
• It should still be a useable and intuitive interface since it's good enough for the sub-packages.
• We don't need to maintain the duplicitous code in 0x.ts, nor any tests for them.

Testing instructions

Types of changes

Checklist:

• Prefix PR title with [WIP] if necessary.
• Prefix PR title with bracketed package name(s) corresponding to the changed package(s). For example: [sol-cov] Fixed bug.
• Add tests to cover changes as needed.
• Update documentation as needed.
• Add new entries to the relevant CHANGELOG.jsons.

[coveralls]

Coverage decreased (-0.2%) to 84.157% when pulling f2d9dfb on refactor/0x-js-exported-interface into fadd292 on development.

[albrow]

🎉 after CI passes and merge conflicts are fixed.

[fabioberger]

Will make this part of my other PR since I used the missing/superfluous export detection feature in there to perfect the index.ts file.