Update R API usage

[coolbutuseless]

Update package to remove use of three non-API calls

• SETLENGTH
• SET_GROWABLE_BIT
• SET_TRUELENGTH

Background

The use of R's API in C code changed last year and has been a blocker on progress - I can't resubmit to CRAN until I deal with the NOTES

I.e.

Found non-API calls to R: ‘SETLENGTH’, ‘SET_GROWABLE_BIT’,
      ‘SET_TRUELENGTH’

I was hoping that either these API calls would be reviewed and eventually allowed in packages, or a decent replacement be proposed by R core. AFAICT neither of those things have happened.