move R includes in a separate file
add context low level stuff not exported by R
start the Context class
C++ level try_catch using contexts
reset_current_error returns a reference, less use of ScalarLogical
only dealing with error conditions
merge from master branch
using internals
resolved conflicts
no need to Shield this SEXP
macro hack so that Shield and DATAPTR play together
more macro stepping over the write barrier
handling PRINTNAME