Hash enc digest hex

[tonesnotes]

The significant change is to the hash classes / functions in regard to the enc (encoding) input argument.

This started from an incorrect comment on PublicKey.verify that the default encoding was 'hex' when it is in fact 'utf8'.

Following this into the hash classes it turned out that the same enc input parameter was being used to determine both the encoding of the input message (msg) AND the desired output type (number[] vs hex string).

As this needed resolving in any case, and it looked like very few tests were passing 'hex' encoding. This PR proposes to resolve the issue by:

1. using the input enc parameter only for msg encoding
2. add an explicit 'utf8' option where missing on the type of the enc parameter
3. add an explicit digestHex() method to mirror digest() when a hex string output is desired.

Two tests were updated. All tests pass.
This is a breaking change to fix a broken API situation.