util: revert some PrefixedHexString breaking changes

[gabrocheleau]

This PR re-adds support for type string as an input type for some util helpers, to preserve backwards compatibility.

I also realized that isHexPrefixed was redundant (we already had an internal helper that implements that behavior) so I removed it.

[scorbajio]

This looks great! Left a few suggestions and comments.

[codecov]

Codecov Report

Attention: Patch coverage is 11.11111% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 80.92%. Comparing base (d24ca11) to head (de9a61b).
Report is 13 commits behind head on master.

Additional details and impacted files

Flag	Coverage Δ
blockchain	90.97% <ø> (ø)
common	?
evm	73.76% <ø> (?)
genesis	99.98% <ø> (?)
statemanager	?
trie	59.42% <20.00%> (?)
util	?
vm	63.00% <0.00%> (?)

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[scorbajio]

ACK