[Rd] Feature request: put NewEnvironment and R_NewhashedEnv into API

[Seth Falcon]

Hi,

I would like to be able to create new environments from package C
code.  AFAICT, this isn't allowed since both NewEnvironment and
R_NewHashedEnv are declared in Defn.R.

If exposing this isn't controvertial, I would submit a patch.
However, I'm not sure where the declarations should go (Rdefines.h?)
and whether they need to change to Rf_*.

+ seth