Size assumptions in parsing code don't hold for regtest blocks

[str4d]

The code below will write an incorrect block if the Equihash parameters are different from mainnet (e.g. for regtest blocks):

https://github.com/zcash-hackworks/lightwalletd/blob/3071b5759ae5b7651d5d13c664e5eb2f5bf503f3/parser/block_header.go#L75-L77

The read side does correctly handle the length:

https://github.com/zcash-hackworks/lightwalletd/blob/3071b5759ae5b7651d5d13c664e5eb2f5bf503f3/parser/block_header.go#L124-L126

[gtank]

For context, "Builder" in this comment refers to a pattern from Go's cryptobyte package, which the internal bytestring type is basically an endian-swapped, bitcoin-aware version of.

[str4d]

SER_BLOCK_HEADER_SIZE will also be incorrect for regtest blocks, which may or may not affect the array allocations (I don't know Golang).

[gtank]

This is definitely why the regtest block hashes weren't as expected.