Edward Z. Yang authored Dec 27, 2016 and Edward Z. Yang committed Dec 27, 2016

There were a number of fields in 'LinkedComponent' which
were "too" flexible, in that they were fully determined by
other fields in the structure.  This refactor deletes those
fields and replaces them with functions that refer to the
fields directly.

I also introduce a new type, ComponentInclude, to take
the place of tuples which were used to represent includes
(mixins) in Backpack.

There's also more documentation for lots of bits.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>