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/syntaxTests/pragma/experimental_solidity.sol

@@ -0,0 +1,3 @@
+pragma experimental solidity;
+// ----
+// Warning 2264: (0-29): Experimental features are turned on. Do not use experimental features on live deployments.

test/libsolidity/syntaxTests/pragma/experimental_solidity_multisource_not_all_enable.sol

@@ -0,0 +1,24 @@
+==== Source: A.sol ====
+contract A {}
+==== Source: B.sol ====
+pragma experimental solidity;
+import "A.sol";
+contract B {
+    A a;
+}
+==== Source: C.sol ====
+pragma experimental solidity;
+import "A.sol";
+contract C {
+    A a;
+}
+==== Source: D.sol ====
+pragma experimental solidity;
+import "A.sol";
+contract D {
+    A a;
+}
+// ----
+// ParserError 2141: (B.sol:0-29): File declares "pragma experimental solidity". If you want to enable the experimental mode, all source units must include the pragma.
+// ParserError 2141: (C.sol:0-29): File declares "pragma experimental solidity". If you want to enable the experimental mode, all source units must include the pragma.
+// ParserError 2141: (D.sol:0-29): File declares "pragma experimental solidity". If you want to enable the experimental mode, all source units must include the pragma.

test/libsolidity/syntaxTests/pragma/experimental_solidity_out_of_order_1.sol

@@ -0,0 +1,5 @@
+contract A {}
+
+pragma experimental solidity;
+// ----
+// ParserError 8185: (45-45): Experimental pragma "solidity" can only be set at the beginning of the source unit.

test/libsolidity/syntaxTests/pragma/experimental_solidity_out_of_order_2.sol

@@ -0,0 +1,13 @@
+function f() pure returns (uint)
+{
+    return 1;
+}
+
+pragma experimental solidity;
+
+struct A
+{
+    uint256 x;
+}
+// ----
+// ParserError 8185: (83-89): Experimental pragma "solidity" can only be set at the beginning of the source unit.