De-duplicate the `Transfer` events in the `_mint` function

[Vectorized]

The events are duplicated on purpose to save gas.

[cygaar]

Perhaps we can add a short comment in the code on why we're duplicating @Vectorized

[KamaDeFi]

I've thought about this some more, and if I understand correctly, the only saving in gas here is to avoid only one extra call of tokenId := add(tokenId, 1) and iszero(eq(tokenId, end)). That doesn't seem to have the massive gas-saving benefit that is worth duplicating code in my opinion.

Examples:

Scenario 1, quantity = 1

Existing code would call:

1. log4
2. let tokenId := add(startTokenId, 1)
3. iszero(eq(tokenId, end))

But the new code would call:

1. let tokenId := startTokenId
2. iszero(eq(tokenId, end))
3. log4
4. tokenId := add(tokenId, 1)
5. iszero(eq(tokenId, end))

Scenario 2, quantity = 2

Existing code would call:

1. log4
2. let tokenId := add(startTokenId, 1)
3. iszero(eq(tokenId, end))
4. log4
5. tokenId := add(tokenId, 1)
6. iszero(eq(tokenId, end))

But the new code would call:

1. let tokenId := startTokenId
2. iszero(eq(tokenId, end))
3. log4
4. tokenId := add(tokenId, 1)
5. iszero(eq(tokenId, end))
6. log4
7. tokenId := add(tokenId, 1)
8. iszero(eq(tokenId, end))