Rename context::token to context::context_index.
Rename graph_roots -> Token
Collect all the Token-related stuff in one place.
Remove unused function context::get_token().
Add some functions for accessing contexts by context_index.

Right now these functions just forward to the token==context_index.
Add function reg_heap::switch_token_to_context( ).
Handle references for tokens differently.
Comment out unused scratch list 'tokens'.
Rename token_roots -> tokens.
Introduce context indirection.
Make context_index immutable.
Move some functions from context into graph_register.
Set modifiable values in terms of contexts.
Remove several functions from context.{H,C}.
Remove context agnostic of tokens.
Add a function for looking up what is called.
Make function more specific.