data.table: use growable vectors

[aitap]

Description

When creating data.table lists or their names, set the GROWABLE_BIT on them. This ensures that R_resizeVector used by data.table won't refuse to work with such data.tables. Drop the finalizer as gc() will count the memory taken by growable vectors correctly.

Main Changes

• call SET_GROWABLE_BIT when setting TRUELENGTH in shallow() and setnames()
• the finalizer is no longer needed

Checklist

• I have performed a self-review of my code.
  • R CMD check passes (except for cryptic vignette problems) with data.table from CRAN and from current master.
• I have commented on my code, particularly in hard-to-understand areas.
• I have updated the documentation where applicable.

Additional Context

Please let me know how you would like to deal with SET_GROWABLE_BIT not being an API entry point. It could be hidden as are other non-API entry points, or the code could make use of R_allocResizableVector and backport it for R < 4.6.

[SebKrantz]

Many thanks @aitap!