Also, some other changes that came up along the way:
- add a 'blk' region for the current block.
- detect unused type/region variables.

- paths can now take region parameters, replacing the dirty hack
  I was doing before of abusing vstores.  vstores are now a bit
  of a hack though.

- fix various small bugs:
  - we never checked that iface types were compatible when casting
    to an iface with `as`
  - we allowed nonsense like int<int>
  - and more! (actually that may be it)

Issue #2201.

Most could use the each method, but because of the hack used to
disambiguate old- and new-style loops, some had to use vec::each.

(This hack will go away soon.)

Issue #1619

and introduce more indirection where it may be needed

Because terr_record_mismatch was getting called by infer::flds,
which takes types a and b where it's trying to prove a <: b, the
expected and actual fields were switched. Fixed it. Closes #2094

- we now distinguish bound/free parameters (see region-param
  test case for why this is necessary)
- we also track bounds on region variables
- also, restructure fold_ty() to have multiple variants without
  duplication instead of one overloaded folder.  This also allows
  for using block functions.

Closes #2114

Also adds proper checking for cont/break being inside a loop.

Closes #1854
Issue #1619