Things are often equivalent but different types:
1. u8 arrays in libwally.
2. sha256
3. Secrets derived via sha256
4. txids
Rather than open-coding a BUILD_ASSERT & memcpy, create a macro to do it.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
And turn "" includes into full-path (which makes it easier to put
config.h first, and finds some cases check-includes.sh missed
previously).
config.h sets _GNU_SOURCE which really needs to be done before any
'#includes': we mainly got away with it with glibc, but other platforms
like Alpine may have stricter requirements.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
