lib: use freelist for WriteWrap instances

WriteWrap JS objects can be re-used across writes, since the associated C++ objects are created and destroyed independently from the JS ones. Since the WriteWrap JS constructor calls into C++, re-using the JS objects instead of creating new ones leads to a minor performance improvement.

At 500 runs:

                                        confidence improvement accuracy (*)   (**)  (***)
 net/net-c2s.js dur=5 type='buf' len=64        ***      1.59 %       ±0.43% ±0.57% ±0.73%

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines