Make arrow mandatory in function types

Author: lukewagner

design/mvp/WIT.md

@@ -340,15 +340,15 @@ parameters, and results. Functions can optionally also be declared as `async`
 functions.
 
 ```wit
-thunk: func()
+thunk: func() -> ()
 fibonacci: func(n: u32) -> u32
-sleep: async func(ms: u64)
+sleep: async func(ms: u64) -> ()
 ```
 
 Specifically functions have the structure:
 
 ```wit
-func-item ::= id ':' 'async'? 'func' func-tuple ( '->' func-tuple )?
+func-item ::= id ':' 'async'? 'func' func-tuple '->' func-tuple
 
 func-tuple ::= ty
              | '(' func-named-type-list ')'
@@ -481,22 +481,22 @@ by '-'s starts with a `XID_Start` scalar value with a zero Canonical Combining
 Class:
 
 ```wit
-foo: func(bar: u32)
+foo: func(bar: u32) -> ()
 
-red-green-blue: func(r: u32, g: u32, b: u32)
+red-green-blue: func(r: u32, g: u32, b: u32) -> ()
 ```
 
 This form can't name identifiers which have the same name as wit keywords, so
 the second form is the same syntax with the same restrictions as the first, but
 prefixed with '%':
 
 ```wit
-%foo: func(%bar: u32)
+%foo: func(%bar: u32) -> ()
 
-%red-green-blue: func(%r: u32, %g: u32, %b: u32)
+%red-green-blue: func(%r: u32, %g: u32, %b: u32) -> ()
 
 // This form also supports identifiers that would otherwise be keywords.
-%variant: func(%enum: s32)
+%variant: func(%enum: s32) -> ()
 ```
 
 [kebab-case]: https://en.wikipedia.org/wiki/Letter_case#Kebab_case