feat: compute buffer size ahead of time

[achingbrain]

The current implementation loops over message fields and each field encoder creates a Uint8Array which is added to a Uint8ArrayList and stitched together at the end of the process.

Other implementations compute the expected length of the serialized message, allocate a buffer that size then have encoders write their data into it at the correct offsets.

This PR makes protons compute the final message buffer size first in the same way.

Thing is, it doesn't seem to make a huge amount of difference to performance.

Before:

Running "Encode/Decode" suite...
Progress: 100%

  pbjs:
    12 166 ops/s, ±3.92%   | 5.12% slower

  protons:
    9 755 ops/s, ±2.19%    | slowest, 23.93% slower

  protobufjs:
    12 823 ops/s, ±2.02%   | fastest

Finished 3 cases!
  Fastest: protobufjs
  Slowest: protons

After:

Running "Encode/Decode" suite...
Progress: 100%

  pbjs:
    11 866 ops/s, ±3.43%   | 2.05% slower

  protons:
    9 356 ops/s, ±2.45%    | slowest, 22.77% slower

  protobufjs:
    12 114 ops/s, ±2.16%   | fastest

Finished 3 cases!
  Fastest: protobufjs
  Slowest: protons

[achingbrain]

Opened as a draft as it's a useful performance comparison but we may not want to merge it.

Longer term we may just want to rip the guts out of another implementation to do the serialization/deserialization as it's the least interesting thing about this module.