tendermint::Hash's FromStr should be case-insensitive

[hdevalence]

Description

Recently I encountered an unintuitive error in pcli: penumbra-zone/penumbra#2092

The cause was that tendermint::Hash's FromStr implementation is case-sensitive, so decoding a lower-case hash failed. We worked around this by uppercasing the string, but it feels like hash parsing should be case-insensitive.

Definition of "done"

Hash parsing should be case-insensitive.

[mzabaluev]

It seems that upper-case has been working for our users since 91c3915. @tony-iqlusion were there any reasons to restrict the hash parsing to uppercase hexadecimals?

[tony-iqlusion]

I'm confused how 91c3915 could've broken anything. It's purely additive?

There was no intent to restrict mixed case or prefer one case over another, it's just unsupported in that PR.