Dedupe ImplementationError

[shinghim]

Closes #647

I opted to create a new core module since there isn't a module to put common/core stuff. The new Version enum in #668 could also be put into this mod, among other things

[coveralls]

Pull Request Test Coverage Report for Build 14744861695

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 82.057%

---

Totals
Change from base Build 14713397893:	0.03%
Covered Lines:	5433
Relevant Lines:	6621

---

💛 - Coveralls

[DanGould]

utACK c2a3082

Decisive move making the core module. We can start moving a bunch of stuff into it, which will reduce the number of feature gates on src/lib.rs