feat: Split name and field_name on Arg object.

[DanCardin]

Fix #46.

At current moment, name field is relatively inaccessible except for the "manual construction" usecase. It's otherwise able to be specified. Thus using such a valuable, generic name seemed unncessary. Thus name -> field_name.

With name freed up, "name" seems like a decent name for the equivalent of metavar in argparse. I suppose name could be confused with short/long?

I looked at Clap which uses value_name, which i rather like, and which seems to me to pair well with field_name. certainly better than metavar

[coveralls]

Pull Request Test Coverage Report for Build 6626228945

• 41 of 41 (100.0%) changed or added relevant lines in 7 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 6618140009:	0.0%
Covered Lines:	1565
Relevant Lines:	1565

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💛 - Coveralls