Add "def" syntax sugar for named function definitions.

This is technically redundant but it makes functional/procedural code
look a *lot* more natural.

test/core/function/call_extra_arguments.wren

@@ -1,7 +1,7 @@
-var f0 = Fn.new { System.print("zero") }
-var f1 = Fn.new {|a| System.print("one %(a)") }
-var f2 = Fn.new {|a, b| System.print("two %(a) %(b)") }
-var f3 = Fn.new {|a, b, c| System.print("three %(a) %(b) %(c)") }
+def f0() { System.print("zero") }
+def f1(a) { System.print("one %(a)") }
+def f2(a, b) { System.print("two %(a) %(b)") }
+def f3(a, b, c) { System.print("three %(a) %(b) %(c)") }
 
 f0("a") // expect: zero
 f0("a", "b") // expect: zero

test/core/function/call_missing_arguments.wren

@@ -1,2 +1,2 @@
-var f2 = Fn.new {|a, b| System.print(a + b) }
+def f2(a, b) { System.print(a + b) }
 f2("a") // expect runtime error: Function expects more arguments.

test/core/function/equality.wren

@@ -11,6 +11,6 @@ System.print(Fn.new { 123 } != false)     // expect: true
 System.print(Fn.new { 123 } != "fn 123")  // expect: true
 
 // Equal by identity.
-var f = Fn.new { 123 }
+def f() { 123 }
 System.print(f == f) // expect: true
 System.print(f != f) // expect: false