Improve write performance of meta API

[d12frosted]

The most obvious way to improve it is to mimic what was done for #64, e.g. allow to pass parsed meta to vulpea-meta-set, but it will not work when adding multiple calls with different props, as meta is modified between the calls, so vulpea-meta-set either should return a modified metadata OR approach this problem in a different fasion - by providing functions to modify meta and to write it.