Tracking issue for type kind representations

Current status of each type and representation

| type & representation | status |
|-------------------------------------------|------------------|
| map {} representation map | Parse error [1] |
| map {} representation stringpairs | Parse error [1] |
| map {} representation listpairs | Parse error [1] |
| struct {} representation map (rename) | OK |
| struct {} representation tuple | OK |
| struct {} representation stringpairs | Not Impl [2] |
| struct {} representation stringjoin | OK |
| struct {} representation listpairs | Not Impl [2] |
| union {} representation keyed | OK |
| union {} representation kinded | OK |
| union {} representation envelope | Not Impl [3] |
| union {} representation inline | Not Impl [3] |
| union {} representation stringprefix | OK |
| union {} representation bytesprefix | Not Impl [3] |
| enum {} representation string | OK |
| enum {} representation int | Bug [4] |

Similar charts here:
https://github.com/ipld/go-ipld-prime/blob/8ad3e7722935154f6fbe09fd495fdafb62af8180/schema/gen/go/README.md

----

### Causes for errors

[1] noname:2:14: unexpected token: "representation"

https://github.com/ipld/go-ipld-prime/blob/master/schema/dsl/parse.go#L561

parser.typeMap doesn't parse the representation. Should be doing:

```
if tok, err := p.peekToken(); err == nil && tok == "representation" {
...
```

[2] noname:5:8: unknown struct repr: "stringpairs"
noname:5:8: unknown struct repr: "listpairs"

https://github.com/ipld/go-ipld-prime/blob/0c00a9390092edcabe37853a19ca9e800633da70/schema/dsl/parse.go#L471

[3] noname:5:8: TODO: union repr "envelope"
noname:5:8: TODO: union repr "inline"
noname:5:8: TODO: union repr "bytesprefix"

https://github.com/ipld/go-ipld-prime/blob/0c00a9390092edcabe37853a19ca9e800633da70/schema/dsl/parse.go#L646

[4] Is parsed and constructed, but t.RepresentationBehavior() == "string" even for "representation int"

root cause unknown