Allow () to be used as error type for nom::bits::{bits,bytes}

[cky]

For most parsers, you can use the unit type as the error type, which is useful if you don't actually care about the details of a parse error, only that it's failed.

However, when using nom::bits::bits and nom::bits::bytes, two error types are required: one for byte-based errors, and one for bit-based errors. Further, the error type has to conform to ErrorConvert, to allow bit-based errors to be converted to byte-based errors (for nom::bits::bits) and vice versa (for nom::bits::bytes).

This branch makes the unit type conform to ErrorConvert with a trivial implementation, to enable it to be used as both error types for nom::bits::bits and nom::bits::bytes.

[coveralls]

Pull Request Test Coverage Report for Build 3807683872

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 61.963%

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Files with Coverage Reduction	New Missed Lines	%
src/traits.rs	5	62.63%

Totals
Change from base Build 3807681115:	0.02%
Covered Lines:	1515
Relevant Lines:	2445

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💛 - Coveralls