support Hindley-Milner polymorphisms in binds and facts;
let_bind(_i): polymorphic version;
moved find_free, export_wrt to Isar/proof_context.ML;
consts ack :: "nat*nat => nat"
recdef ack "measure(%m. m) ** measure(%n. n)"
"ack(0,n) = Suc n"
"ack(Suc m,0) = ack(m, 1)"
"ack(Suc m,Suc n) = ack(m,ack(Suc m,n))"