Commit 759be3dad9 bumped our Go
requirement to 1.18 which allows us to drop support for old-style
`+build` tags. This change runs `go fix ./...` to remove them, and
updates some remaining code generators that produced `+build` lines.
Adds a test for function signature memory layout by generating functions with
random signatures. This confirms that the size and offsets computed by
`gotypes` agree with `asmdecl`.
Updates #191#195