TX_SKIP message calls

The spec says that one should keep warmths for TX_SKIP as they are. I wonder what that would mean e.g. if

• sender is a precompile (does our testing environment even allow this ? or would we need to produce a signature for the (unknown) private key of a precompile) ?
• coinbase is a precompile (those are warm by default)

N.B. Recipients may not be precompiles.

source #1510 (comment)

TX_SKIP deployment transactions

If we don't have them yet we should add tests for TX_SKIP with trivial deployments. We can play the same games of

• sender is coinbase
• (deployment address) is coinbase
• coinbase is a precompile

source #1510 (comment)