ClassfileParser: Java8 bytecode (fixes #83)

I went through all commits in

    git log v2.10.0..v2.10.4-RC3 --oneline -- src/compiler/scala/tools/nsc/symtab/classfile

and applied them to dotty where possible. Running

    bin/dotc tests/pos/test.scala -java-home /usr/lib/jvm/jre-1.8.0

now succeeds, no more errors like in https://github.com/lampepfl/dotty/issues/83