polish zmtp frame toString
frame toString: do not modify reader index
encoder: store target buffer size in variable
encoder: do not modify reader index when writing
frame: no buffer instantiation on data access

Avoid garbage generation and buffer initialization
at the cost of requiring more careful frame buffer
use.

Also allow byte array access by not returning a
read only buffer.
optimize parser input fragmentation test
utils: polish message size calculation
release 0.1.2
start 0.1.3 development