Add lyd_print_mem_len API to return printed size

[irfanHaslanded]

It can be useful to know how many bytes were written, and avoid an expensive and redundant strlen by the application.

[michalvasko]

You can write such a wrapper for your application, if you need it, just use lyd_print_all(), for example, instead of lyd_print_() so unless you have some arguments supporting adding this API, I will close this PR.

[irfanHaslanded]

I see, similar to sr_lyd_print_lyb. Thanks Michal