Adjust binary encoding of import/export names (#623)

* Adjust binary encoding of import/export names

The purpose of this commit is to address the discrepancy between the
specification and wasm-tools discovered in #622. The history here is:

* #222 - historical addition of `(interface "name")` as an import/export
  name.
* #263 - moral revert of the previous change where the kind of name was
  now encoded in the name instead. At this point `Binary.md` mistakenly
  no longer reflected what wasm-tools parsed.
* #536 - addition of more metadata to import/export names with a
  discriminator byte that overlapped the previous.

This commit is is an adjustment to `Binary.md` to fix these
discrepancies. Notably `exportname` and `importname` productions,
previously absent. The `exportname'` and `importname'` productions are
now renamed to `exportname` and `importname`. Additioanlly the
production with `versionsuffix'` have been renumbered to `0x02` while an
`0x01` production was added which is the exact same as `0x00` (intended
for historical compat). A note was updated in the 1.0 binary adjustments
to clean this up.

Closes #622

* Review comments

* Update design/mvp/Binary.md

Co-authored-by: Luke Wagner <mail@lukewagner.name>

---------

Co-authored-by: Luke Wagner <mail@lukewagner.name>

Author: alexcrichton

design/mvp/Binary.md

@@ -389,10 +389,12 @@ in the explainer.)
 import         ::= in:<importname'> ed:<externdesc>                     => (import in ed)
 export         ::= en:<exportname'> si:<sortidx> ed?:<externdesc>?      => (export en si ed?)
 importname'    ::= 0x00 len:<u32> in:<importname>                       => in     (if len = |in|)
-                 | 0x01 len:<u32> in:<importname> vs:<versionsuffix'>   => in vs  (if len = |in|) 🔗
-exportname'    ::= 0x00 len:<u32> en:<exportname>                       => en     (if len = |en|)
-                 | 0x01 len:<u32> en:<exportname> vs:<versionsuffix'>   => in vs  (if len = |in|) 🔗
-versionsuffix' ::= len:<u32> vs:<semversuffix>                          => (versionsuffix vs)  (if len = |vs|) 🔗
+                 | 0x01 len:<u32> in:<importname>                       => in     (if len = |in|)
+                 | 0x02 len:<u32> in:<importname> vs:<versionsuffix>    => in vs  (if len = |in|) 🔗
+exportname'    ::= 0x00 len:<u32> in:<exportname>                       => in     (if len = |in|)
+                 | 0x01 len:<u32> in:<exportname>                       => in     (if len = |in|)
+                 | 0x02 len:<u32> in:<exportname> vs:<versionsuffix>    => in vs  (if len = |in|) 🔗
+versionsuffix  ::= len:<u32> vs:<semversuffix>                          => (versionsuffix vs)  (if len = |vs|) 🔗
 ```
 
 Notes:
@@ -406,6 +408,8 @@ Notes:
   of the inferred `externdesc` of the `sortidx`.
 * `<importname>` and `<exportname>` refer to the productions defined in the
   [text format](Explainer.md#import-and-export-definitions).
+* `<importname'>` and `<exportname'>` will [be cleaned up for a 1.0
+  release](##binary-format-warts-to-fix-in-a-10-release).
 * The `<importname>`s of a component must all be [strongly-unique]. Separately,
   the `<exportname>`s of a component must also all be [strongly-unique].
 * Validation requires that annotated `plainname`s only occur on `func` imports
@@ -519,9 +523,9 @@ named once.
 * The two `depname` cases should be merged into one (`dep=<...>`)
 * The two `list` type codes should be merged into one with an optional immediate
   and similarly for `func`.
-* The `0x00` variant of `importname'` and `exportname'` will be removed. Any
-  remaining variant(s) will be renumbered or the prefix byte will be removed or
-  repurposed.
+* The `0x00` and `0x01` variant of `importname'` and `exportname'` will be
+  removed. Any remaining variant(s) will be renumbered or the prefix byte will
+  be removed or repurposed.
 * Most built-ins should have a `<canonopt>*` immediate instead of an ad hoc
   subset of `canonopt`s.
 * Add optional `shared` immediate to all canonical definitions (explicitly or