Add range loops to spec

design/spec.md

@@ -276,6 +276,12 @@ statement. The result of the loop may be assigned to any type that satisfies the
 assignment rules of all of its break statements. Loops may be nested, and break
 statements only apply to the closest containing loop. The value of the loop's
 expression is never used.
+### For
+For is a special kind of loop that evaluates an expression for each element of
+an array or slice. It accepts an index declaration and an element declaration,
+which are scoped to the loop's body and are set to the index of the current
+element and the element itself respectively at the beginning of each iteration.
+The assignment rules of a for statement are identical to that of a normal loop.
 ### Break
 Break allows breaking out of loops. It may be assigned to anything, but the
 assignment will have no effect as execution of the code after and surrounding it
@@ -362,6 +368,7 @@ this without hand-writing a parser.
                  ["else" <expression>]
 <match>        -> "match" <expression> <case>*
 <loop>         -> "loop" <expression>
+<for>          -> "for" <expression> [<expression>] in <expression> <expression>
 <break>        -> "[" "break" [<expression>] "]"
 <return>       -> "[" "return" [<expression>] "]"
 <assignment>   -> <expression> "=" <expression>