Add blobs and shorthand pmt pseudo-constructors.

Add blobs and shorthand pmt pseudo-constructors.

blobs == Binary Large Object.  Very handy for passing around
uninterpreted data.  The shorthand constructors were implemented by
overloading the pmt_t mp(foo) function in the pmt namespace.

I originally called "mp" "pmt", but that caused a conflict with the
pmt namespace.