move OpenJDK.getProtectionDomain into Classes.java

This way, apps can access the CodeSource of a class whether they're
using the OpenJDK class library or the Avian one.
Merge pull request #183 from dicej/codesource

move OpenJDK.getProtectionDomain into Classes.java
remove redundant decrement in ZipFile.getInputStream inner class

We were decrementing the "remaining" field twice for each byte read
using the no-arg read method, which resulted in available() returning
a value that was too small.
call vm::dynamicCall directly, instead of through System
Merge pull request #182 from joshuawarner32/ffi-split

split out ffi::call from System
Merge pull request #184 from dicej/zip-available

remove redundant decrement in ZipFile.getInputStream inner class