Drop TLS dep entirely

[AlCutter]

This PR removes the dependency on the 3rd party TLS encoding lib.

TLS encoding is only used to serialise the STH preimage for verifying its signature, this is actually a pretty simple case since all the components are fixed sizes - there's no need for the extra complexity which comes from a fully featured TLS encoder, so we can just implement it inline.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.24%. Comparing base (fa00c16) to head (fa5b285).
Report is 65 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #150      +/-   ##
==========================================
- Coverage   82.57%   77.24%   -5.33%     
==========================================
  Files           5        6       +1     
  Lines         241      334      +93     
==========================================
+ Hits          199      258      +59     
- Misses         30       45      +15     
- Partials       12       31      +19     

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[mhutchinson]

Nice.