Various minor fixes and changes.
Various minor fixes and changes.

* 'nm' prefix changed to 'nma' to be in accord with the draft
* @nma:ordered-by is now attached to <element>, not <...OrMore>.
* Implemented 'mandatory' and 'default' under 'choice'.
* Updated 'default' under 'typedef' and 'leaf'
* 'must' condition is @nma:assert attr., not element

vkosuri/pyang,mbj4668/pyang,jgatkinsn/pyang,kspviswa/pyang,vkosuri/pyang,krvinay123/test,jgatkinsn/pyang,acidjunk/pyang,brenoah/pyang,krvinay123/test,brenoah/pyang,mbj4668/pyang,davidegurgone/pyang,acidjunk/pyang,davidegurgone/pyang,kspviswa/pyang