txscript: Separate code for standard scripts.

This commit moves all code related to standard scripts into a separate
file name standard.go as well as the associated tests into
standard_test.go.  Since the code in address.go and address_test.go is
only related to standard scripts, it has been combined into the new
files and the old files deleted.

The intent here is to make it clear that the code in standard.go is not
related to consensus.