This brings in the new HCL extension functions "try", "can", and
"convert", along with the underlying HCL and cty infrastructure that allow
them to work.
This is largely minor bugfixes for issues found since we last updated the
vendoring. There are some new features here that Terraform is not yet
using and thus present little risk.
In particular this includes the HCL-JSON spec change where arrays can now
be used at any level in a block label structure, to allow for preserving
the relative order of blocks.
Alisdair McDiarmid