Require `then` and `else` with `if` (#6201)

We previously supported (and primarily used) a non-standard text format for
conditionals in which the condition, if-true expression, and if-false expression
were all simply s-expression children of the `if` expression. The standard text
format, however, requires the use of `then` and `else` forms to introduce the
if-true and if-false arms of the conditional. Update the legacy text parser to
require the standard format and update all tests to match. Update the printer to
print the standard format as well.

The .wast and .wat test inputs were mechanically updated with this script:
https://gist.github.com/tlively/85ae7f01f92f772241ec994c840ccbb1

2 files changed, 12 insertions, 6 deletions

diff --git a/test/ctor-eval/partial-return.wast b/test/ctor-eval/partial-return.wast
index 8ca6eee75..e74c59cb8 100644
--- a/test/ctor-eval/partial-return.wast
+++ b/test/ctor-eval/partial-return.wast
@@ -18,7 +18,9 @@
       (i32.load8_u
         (i32.const 12)
       )
-      (return)
+      (then
+        (return)
+      )
     )
 
     ;; This is unsafe to call, and would stop evalling here. But we exit due to
diff --git a/test/ctor-eval/results.wast b/test/ctor-eval/results.wast
index bfefa2756..dcd10ea5f 100644
--- a/test/ctor-eval/results.wast
+++ b/test/ctor-eval/results.wast
@@ -41,13 +41,17 @@
     ;; we should succeed. After that we should keep returning the constant 55
     (if (result i32)
       (i32.const 1)
-      (block (result i32)
-        (global.set $global4
-          (i32.const 14)
+      (then
+        (block (result i32)
+          (global.set $global4
+            (i32.const 14)
+          )
+          (i32.const 55)
         )
-        (i32.const 55)
       )
-      (i32.const 99)
+      (else
+        (i32.const 99)
+      )
     )
   )