Fix cpp11 rchk

[astamm]

Everything can be found in r-lib/cpp11#456.

For the time being, I vendored {cpp11} headers and patched them. Specifically:

Fix for insert() in protect.hpp.

The key changes were to remove PROTECT/UNPROTECT calls since:

• list from get() is already preserved;
• The cons cell is part of the preserve list structure;
• The input x is protected by the caller.

Fixes in r_vector.hpp

Key changes to reserve_data()

1. Removed all explicit PROTECT/UNPROTECT calls since protection is handled by the safe wrapper in both resize_data and resize_names()
2. Simplified the flow of the names handling
3. Let R's garbage collection handle the intermediate objects

Key change to resize_data()

1. Moving both the PROTECT and UNPROTECT calls since the safe wrapper around Rf_allocVector already handles protection.

Key changes to resize_names()

1. The safe wrapper around Rf_allocVector already handles protection
2. Removed direct pointer access via STRING_PTR_RO
3. Use STRING_ELT instead of direct pointer access