py/vstr: Change allocation policy, +16 to requested size, instead of *2.

Effect measured on esp8266 port:

Before:
>>> pystone_lowmem.main(10000)
Pystone(1.2) time for 10000 passes = 44214 ms
This machine benchmarks at 226 pystones/second
>>> pystone_lowmem.main(10000)
Pystone(1.2) time for 10000 passes = 44246 ms
This machine benchmarks at 226 pystones/second

After:
>>> pystone_lowmem.main(10000)
Pystone(1.2) time for 10000 passes = 44343ms
This machine benchmarks at 225 pystones/second
>>> pystone_lowmem.main(10000)
Pystone(1.2) time for 10000 passes = 44376ms
This machine benchmarks at 225 pystones/second