Add links to the new constant expressions section

Author: matthewjasper

src/items.md

@@ -715,12 +715,14 @@ More detailed specification for unions, including unstable bits, can be found in
 
 ## Constant items
 
-A *constant item* is a named _constant value_ which is not associated with a
+A *constant item* is a named _[constant value]_ which is not associated with a
 specific memory location in the program. Constants are essentially inlined
 wherever they are used, meaning that they are copied directly into the relevant
 context when used. References to the same constant are not necessarily
 guaranteed to refer to the same memory address.
 
+[constant value]: expressions.html#constant-expressions
+
 Constant values must not have destructors, and otherwise permit most forms of
 data. Constants may refer to the address of other constants, in which case the
 address will have elided lifetimes where applicable, otherwise – in most cases –

src/tokens.md

@@ -12,7 +12,8 @@ grammar as double-quoted strings. Other tokens have exact rules given.
 A literal is an expression consisting of a single token, rather than a sequence
 of tokens, that immediately and directly denotes the value it evaluates to,
 rather than referring to it by name or some other evaluation rule. A literal is
-a form of constant expression, so is evaluated (primarily) at compile time.
+a form of [constant expression](expressions.html#constant-expressions), so is
+evaluated (primarily) at compile time.
 
 ### Examples