* add rustc_version to build deps of ether types (needed by build script)
* ethereum-types: add build script that sets cargo:rustc-cfg=asm_available on nightly
* ethereum-types: #![cfg_attr(asm_available, feature(asm))]
* tests: add the whole feature asm build script to make it pass on nightly CI
* add #![cfg_attr(asm_available, feature(asm))] directly where needed
so users don't have to add it
* remove tests/build.rs since it should not be needed anymore
* move inner cfg_attr attributes to the right allowed position
* remove check for asm_available in contexts that already have check for that
snd