Pre-allocate in as_ssz_bytes

[macladson]

During encoding, we can pre-allocate the output buffer since we already know the final SSZ length.
This avoids potential re-allocations as the buffer grows during encoding.

From my rough testing, this typically saves around 5-10% with gains up to 50% depending on how much allocation and re-allocation overhead dominates the encoding time

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.39%. Comparing base (d3ecadb) to head (7bf8e11).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #82      +/-   ##
==========================================
+ Coverage   82.36%   82.39%   +0.03%     
==========================================
  Files          13       13              
  Lines        1140     1142       +2     
==========================================
+ Hits          939      941       +2     
  Misses        201      201              

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