lib: fix implicit seqof encoding

Following schema encoding is broken:

[0] IMPLICIT SEQOF Attribute

Implicit tag is prepended to wrapping SEQ and to every element
inside it.

Implicit state should be reseted for SEQOF children
Don't cache submodels

asn1.js uses same model object for different depths of
recursive encoding.

It is possible that use-function would return different
submodels on different calls so caching it's return value would
break encoding.