add _unsafeMint function

[ahbanavi]

For easier use so that we do not have to fill all 4 parameters.
I think with that way, mocking and testing _mint function is redundant, let me know if it did, so I remove related tests.

Also, I think _mint function should be private instead of internal cause with _safeMint and _unsafeMint functions there's no additional need for developers to use this method directly. I wait to hear other's opinion about this.

I will add testGas tests after I figure out whether we should make _mint function internal or make it private. Also, there are some bugs in test/GasUsage.test.js context names, I'm not sure to fix them in this PR or make another.

[syffs]

I don't think it's worth adding a wrapper around _mint function (which is going to add more gas on the deployment) for the reasons you mentioned

[cygaar]

Agree with @syffs here, don't think this adds too much value, and I prefer not to add more code just to save on adding in two parameters to a function call

[ahbanavi]

Makes sense, I open another PR later for fixes on test/GasUsage.test.js.