Fix coinbase height deserialization

[oxarbitrage]

Motivation

In height deserialization we are allowing more than the spec. This will close #2226 if merged.

Specifications

A coinbase transaction for a block at block height greater than 0 MUST have a script that, as its first item, encodes the block height height as follows.

For height in the range {1 .. 16}, the encoding is a single byte of value 0x50 + height.

Otherwise, let heightBytes be the signed little-endian representation of height, using the minimum nonzero number of bytes such that the most significant byte is < 0x80. The length of heightBytes MUST be in the range {1 .. 5}. Then the encoding is the length of heightBytes encoded as one byte, followed by heightBytes itself.

This matches the encoding used by Bitcoin in the implementation of [BIP-34] (but the description here is to be considered normative).

Solution

Constrain the decoding by block height ranges. Added some tests.

Review

@teor2345 is probably the most familiar with this but anyone can take a look.

Reviewer Checklist

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

[teor2345]

Looks good, but I think a few more tests would help.

[teor2345]

Almost there, we just need to cover all the possible encoded byte formats in the tests.

[teor2345]

Looks great, thanks for writing all those tests!