make and use ENCLOS(R_EmptyEnv) == R_EmptyEnv)
Merge branch '23-more-constants' into 24-misc
put ENCLOS(R_EmptyEnv) back to R_NilValue
Merge branch '23-more-constants' into 24-misc
have forcePromise protect its arguments
define and use IS_BASE, put some of FindVarInFrame3 into findFun
more use of forcePromise and isFunction