Added constants to the spec

design/spec.md

@@ -12,7 +12,8 @@ are:
 - Private access: Disallows other modules from accessing a top-level entity.
   This mode is the default when one isn't supplied.
 ### Type definition
-Type definitions bind a type to a global type identifier.
+Type definitions bind a type to a global type identifier. They may list named
+constant values of their type accessible via constant access.
 ### Function
 Functions bind a global identifier and argument list to an expression which is
 evaluated each time the function is called. If no expression is specified, the
@@ -303,6 +304,11 @@ the function or method will cease.
 Assignment allows assigning the result of one expression to one or more location
 expressions. The assignment expression itself has no value and may not be
 assigned to anything.
+### Constant access
+Constant access allows accessing an enumerated constant defined as part of a
+type. It can be assigned to a type matching the constant declaration's type.
+Since it contains inherent type information, it may be directly assigned to an
+interface.
 
 # Syntax entities