ERC721 bugfix + gas optimizations

[nventuro]

This removes the _addTokenTo and _removeTokenFrom internal functions (which had a notice indicating they were not meant to be used, and only existed due to a language limitation, and are therefore not considered part of the API). They have been inlined into _mint and _burn, making it easier to understand how each function is called in ERC721Enumerable.

Additionally, #839 had already been accidentally fixed in #1539, but the current implementation makes it more obvious that the issues is gone.

Fixes #839
Closes #912
Closes #1541

[nventuro]

I just noted _burn has the same swap and pop behavior, and therefore contains its own copy of #839: I'll follow up on this and fix that.

[nventuro]

Coverage dropped due to a check that is now in _burn and used to be tested via _removeTokenFrom in the tests I got rid of, but that require won't make sense after #1550 goes in - we could first merge that PR and then this one on top.

[frangio]

Merged #1550 and restarted the build.

[nventuro]

Hmm, I guess we really can't get rid of that require. We'll need to add a public method on a mock to explicitly test it.

[nventuro]

I know the _mint test is sort of out of scope, but the _burn tests require _mint, and having that doesn't really make sense unless _mint is also tested.

[nventuro]

Should be ready now, @frangio PTAL. The functions for adding/removing tokens from both enumerations should be replaced by a function that receives a storage mapping (they are pretty much the same), but sadly that isn't supported in 0.4.x.

[frangio]

Nice work. 😊