compile: asm and core options

Implement the suggestions I made in http://erlang.org/pipermail/erlang-questions/2013-August/075161.html:

* compile(3): stop declaring 'asm' as "for internal debugging use"
* compile:file/2: implement 'core' option
* compile_SUITE: test 'asm'
* compile:compile_core/3: reuse new 'core' option
* compile(3): document 'from_asm' and 'from_core'