Provide a SBFrame.register/.reg helper on the Python side of SBFrame to vend a flattened view of registers

Our actual view of registers is a set of register sets, each one of which contains a subset of the actual registers
This makes trivial scripting operations tedious ("I just want to read r7!")

This helper allows things like: print lldb.frame.reg["r7"]

Fixes rdar://19185662