Adding -asap-warn-unsafecasts=true option to the typechecker
Made a very basic test for unsafe casts warning (it needs to be expanded at some point).
Fixed small issues in Typechecker related to warnings about implicit & explicit casts.
Modified checker to take command a line argument -asap-default-scheme to
slect which annotation scheme to use as a default, instead of having
a different checker for each of them.