Borrowing a fix from another fork of FakeIt to fix colliding mocking ids

[AlexTroyer]

We have seen issues where mocked methods defined in different files will end up overwriting each other. This was because the identifier for the mocked methods was defined by the __COUNTER__ macro, which increments per compilation unit

This change comes from another fork of FakeIt that looks like it will merge at some point to the main repo

[coveralls]

Pull Request Test Coverage Report for Build 4398645576

• 8 of 8 (100.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-4.0e**-05%**) to 99.925%

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Totals
Change from base Build 4287243006:	-4.0e-05%
Covered Lines:	4009
Relevant Lines:	4012

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💛 - Coveralls

[nickgra]

Upstream PR is eranpeer/FakeIt#296