Implement intelligent breaking for poly-var type expressions

Fixes #245

Given:
```rescript
type currencyPoly = [#USD
  | #CAD | #EUR]

type currencyPoly = [#USD | #CAD | #EUR]
```
we have two different layouts:

In the first example the user wrote them over multiple lines.
The second has them on one line.
This commit implements a new strategy to print these two examples.
We now look at the style of the author: did he write it over multiple lines or not?

If it is on one line, keep it on one line (unless it breaks the column width).
If it is written over multiple lines, force break it over multiple lines.
This is also consistent with how we print variant declaration.

src/res_printer.ml

@@ -1587,6 +1587,7 @@ and printTypExpr (typExpr : Parsetree.core_type) cmtTbl =
   | Ptyp_class _ ->
     Doc.text "classes are not supported in types"
   | Ptyp_variant (rowFields, closedFlag, labelsOpt) ->
+    let forceBreak = typExpr.ptyp_loc.Location.loc_start.pos_lnum < typExpr.ptyp_loc.loc_end.pos_lnum in
     let printRowField = function
     | Parsetree.Rtag ({txt}, attrs, true, []) ->
       Doc.group (
@@ -1615,7 +1616,10 @@ and printTypExpr (typExpr : Parsetree.core_type) cmtTbl =
     in
     let docs = List.map printRowField rowFields in
     let cases = Doc.join ~sep:(Doc.concat [Doc.line; Doc.text "| "]) docs in
-    let cases = if docs = [] then cases else Doc.concat [Doc.ifBreaks (Doc.text "| ") Doc.nil; cases] in
+    let cases =
+      if docs = [] then cases
+      else Doc.concat [Doc.ifBreaks (Doc.text "| ") Doc.nil; cases]
+    in
     let openingSymbol =
       if closedFlag = Open
       then Doc.concat [Doc.greaterThan; Doc.line]
@@ -1628,10 +1632,14 @@ and printTypExpr (typExpr : Parsetree.core_type) cmtTbl =
     | Some([]) ->
       Doc.nil
     | Some(labels) ->
-      Doc.concat (List.map (fun label -> Doc.concat [Doc.line; Doc.text "#" ; printPolyVarIdent label] ) labels)
+      Doc.concat (
+        List.map (fun label ->
+          Doc.concat [Doc.line; Doc.text "#" ; printPolyVarIdent label]
+        ) labels
+      )
     in
     let closingSymbol = if hasLabels then Doc.text " >" else Doc.nil in
-    Doc.group (
+    Doc.breakableGroup ~forceBreak (
       Doc.concat [
         Doc.lbracket;
         Doc.indent (

tests/printer/typexpr/__snapshots__/render.spec.js.snap

@@ -596,7 +596,10 @@ let id = (x: [> #Red | #Green | #Blue]) => x
 let upper = (x: [< #Red | #Green]) => true
 type point = [#Point(float, float)]
 type \\\\\\"type\\" = [#\\"Point🗿\\"(\\\\\\"let\\", float)]
-type shape = [#Rectangle(point, point) | #Circle(point, float)]
+type shape = [
+  | #Rectangle(point, point)
+  | #Circle(point, float)
+]
 
 type madness = [< #\\"type\\" & (\\\\\\"let\\") & (\\\\\\"Super exotic\\") | #\\"Bad Idea\\"]
 
@@ -648,7 +651,11 @@ type x = [
   | #Baaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaz
 ]
 
-type animation = [#\\"ease-in\\" | #\\"ease-out\\" | #\\"never ease ✍️\\"]
+type animation = [
+  | #\\"ease-in\\"
+  | #\\"ease-out\\"
+  | #\\"never ease ✍️\\"
+]
 
 module type Conjunctive = {
   type u1 = [#A | #B]
@@ -663,5 +670,22 @@ module type Conjunctive = {
     & ([< #\\"Exotic-u1+++\\"])
   ] => unit
 }
+
+// should break because user wrote it over serveral lines
+type currencyPoly = [
+  | #USD
+  | #CAD
+  | #EUR
+]
+
+// should not break, user wrote it on one line
+type currencyPoly = [#USD | #CAD | #EUR]
+//
+// should break, line length exceeded
+type currencyPoly = [
+  | #UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUSD
+  | #CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAD
+  | #EUUUUUUUUUUUUUUUUUUUR
+]
 "
 `;

tests/printer/typexpr/variant.js

@@ -82,3 +82,14 @@ module type Conjunctive = {
   let g: [< | #S&([< u2]) & ([< u2]) & ([< u1])] => unit
   let g: [< | #"Exotic-S+"&([< #"Exotic-u2+"]) & ([< #"Exotic-u2-"]) & ([< #"Exotic-u1+++"])] => unit
 };
+
+
+// should break because user wrote it over serveral lines
+type currencyPoly = [#USD 
+  | #CAD | #EUR]
+
+// should not break, user wrote it on one line
+type currencyPoly = [#USD | #CAD | #EUR]
+//
+// should break, line length exceeded
+type currencyPoly = [#UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUSD | #CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAD | #EUUUUUUUUUUUUUUUUUUUR]