the ExtLam constructor does pop up in a few places during reification

substitution and free variable computation in particular
disable display forms and extra unfolding when reifying in unquote
Unquote AGDATERMUNSUPPORTED to meta instead of raising an error
[ issue1228 ] better error when mixing up con and def in unquote
[ issue1228 ] more principled checking of name kinds when unquoting
[ issue1228 ] oops, recursive unquoteDecls have no definitions
[issue 938] implement recursive instance search (without termination check)