fix(chain): make FromHex consistent with ToHex for tx/block hashes

[conradoplg]

Motivation

FromHex wasn't reversing the bytes like ToHex does for transaction and block hashes. (See here for some background on this terrible decision of reversing in the first place 🤣 )

Specifications

Designs

Solution

Fix it, add tests

Review

Detected while working on #3145, also discovered in #3891. It's not currently blocking those, but will soon.

Reviewer Checklist

• Code implements Specs and Designs
• Tests for Expected Behaviour
• Tests for Errors

Follow Up Work

[oxarbitrage]

Looks good, thanks!

[conradoplg]

lightwalletd test failed with

Error: 
   0: stdout of command did not contain any matches for the given regex

Location:
   /app/zebra-test/src/command.rs:330

Command:
   "lightwalletd" "--zcash-conf-path" "/tmp/zebrad_testsTk0gns/lightwalletd-zcash.conf" "--data-dir" "/tmp/zebrad_testsTk0gns" "--log-file" "/dev/stdout" "--grpc-bind-addr" "127.0.0.1:0" "--http-bind-addr" "127.0.0.1:0" "--no-tls-very-insecure"

Exit Status:
   command terminated unsuccessfully by signal 9

Match Regex:
   "Ingestor adding block to cache"

Command:
   "/app/target/release/zebrad" "-c" "/tmp/zebrad_testshcRZTZ/zebrad.toml" "start"

Exit Status:
   command terminated unsuccessfully by signal 9

Stderr:
   [sentry] initialized disabled sentry client due to disabled or invalid DSN

Works on my machine though, so I'm retrying it