Change data representation of resource methods

[alexcrichton]

Note: this builds on #1068

This commit changes the representation of resources and their methods in
the wit-parser crate as implemented originally in #1053. Previously
methods related to a resource were strung as a separate list of methods
from each Resource type which provided a nested view of functions for
bindings generators to interpret. This representation, however, does not
allow for methods or constructors to actually refer to the resource type
itself because the resource type can't be created until the method list
is created, a cyclic dependency.

To handle this problem this commit removes the nested list of functions
and instead places all resource methods and functions and such within
the normal single list of functions within an interface. This represents
a "flattened" version of all functions available within an interface or
world and aligns with the WebAssembly representation of how these will
work. The names of these functions are not valid identifiers and have
the component-model mangling along the lines of [method]foo.bar for
example.

My hope is that in the future as bindings generators are worked on
various helper methods can be added to automatically group functions
based on their kind to avoid any issues. Otherwise though this will
likely "break" all existing code generators when resources are
introduced because the names aren't valid identifiers in any language.
I think this is a good thing, however, as that way it can't be forgotten
to handle the resources case and failures will be "loud" in theory.

This commit changes a number of other miscellaneous details such as:

• Type resolution no longer needs to consider types referenced by
  methods of resources as dependencies which reflects how this will all
  look in a wasm binary format where resources are declared before their
  member functions are declared.

• Parsing has been updated to address some minor issues here and there
  and additionally has some new names to remove the old "value" concept
  which is no longer a thing.