fixes a crash in ReflectionUtils.systemProperties

Due to SI-7465 we were erroneously assuming that System.getProperties
only contains string key and string values. This led to a CCE when there
was something else.
removes the traces of always on debug diagnostics

pun intended
replaces inferBootClasspath with a simple lookup at sun.boot.class.path

It's not like we're achieving any generality by iterating through all
keys in System.getProperties and looking for ones which resemble
"boot.class.path", so let's be simpler.
pull request feedback
Merge pull request #2505 from scalamacros/topic/macros

some fixes for macros: one esoteric, and one quite critical