Use Growable Vectors in MCMC in BAS to reduce memory

[merliseclyde]

As a way to eliminate allocation of more memory than needed, use growable vectors to reduce memory footprint growing as needed.

See the function EnlargeVector as a prototype to use initially and possibly improve for memory management (use memcpy and memset instead of a loop for certain classes where feasible). A version without SEXP types that uses R_Realloc could be more efficient in terms of memory, but need to check on how the expanded space is initialized, per certain CRAN checks.

This should eliminate the use of SETLENGTH (see issue #82) via xlengthgets.