Blockheader::spv_validate() is misleadingly named

[petertodd]

This doesn't even properly validate PoW, let alone "SPV", as it's simply a single-block validation.

A better name would be validate_target(), as it validates that the required target has been met. To fix this I'd suggest that we just change the name, and add a #inline(always) wrapper for the old name that's marked depreciated.

[apoelstra]

Agreed. This is a holdover from the ancient src/blockdata/blockchain.rs days (where there was appropriate supporting code to actually compute the expected difficulty). This was removed in March and perhaps this function should've been removed too. Oh well.

The old supporting code was moved to https://github.com/tamasblummer/rust-bitcoin-chain which AFAIK is the only place this function is used.