compile(3): stop declaring 'asm' as "for internal debugging use"
compiler_SUITE: fix minor typos in comments
compile:file/2: implement 'core' option

Reuse existing test for 'from_core' by changing the code to use 'core'
instead. 'from_core' is tested in various other tests.
compile_SUITE: test 'asm'

Reuse existing test for 'from_asm' by changing the code to use 'asm'
instead. 'from_asm' is tested in various other tests.
compile:compile_core/3: reuse new 'core' option

compile_asm/3 uses 'asm' instead of 'from_asm'. Do the same by using
'core' in compile_core/3.
compile(3): document 'from_asm' and 'from_core'