Introduce solidity-next pragma

Exclude pragma experimental error from ANTLR tests

Test for first pragma after non-pragma declaration

Resolve import pragmas

Change pragma name from next to solidity

Add Changelog entries

Address review comments

test/libsolidity/ASTJSON/pragma_experimental_solidity.json

@@ -0,0 +1,21 @@
+{
+  "absolutePath": "a",
+  "experimentalSolidity": true,
+  "exportedSymbols": {},
+  "id": 2,
+  "nodeType": "SourceUnit",
+  "nodes":
+  [
+    {
+      "id": 1,
+      "literals":
+      [
+        "experimental",
+        "solidity"
+      ],
+      "nodeType": "PragmaDirective",
+      "src": "0:29:1"
+    }
+  ],
+  "src": "0:30:1"
+}

test/libsolidity/ASTJSON/pragma_experimental_solidity.sol

@@ -0,0 +1,3 @@
+pragma experimental solidity;
+
+// ----

test/libsolidity/ASTJSON/pragma_experimental_solidity_parseOnly.json

@@ -0,0 +1,20 @@
+{
+  "absolutePath": "a",
+  "experimentalSolidity": true,
+  "id": 2,
+  "nodeType": "SourceUnit",
+  "nodes":
+  [
+    {
+      "id": 1,
+      "literals":
+      [
+        "experimental",
+        "solidity"
+      ],
+      "nodeType": "PragmaDirective",
+      "src": "0:29:1"
+    }
+  ],
+  "src": "0:30:1"
+}