Introduce abicoder pragma.

test/libsolidity/syntaxTests/abiEncoder/conflicting_settings.sol

@@ -0,0 +1,4 @@
+pragma experimental ABIEncoderV2;
+pragma abicoder v1;
+// ----
+// SyntaxError 3845: (34-53): ABI coder has already been selected for this source unit.

test/libsolidity/syntaxTests/abiEncoder/conflicting_settings_reverse.sol

@@ -0,0 +1,4 @@
+pragma abicoder v1;
+pragma experimental ABIEncoderV2;
+// ----
+// SyntaxError 8273: (20-53): ABI coder V1 has already been selected through "pragma abicoder v1".

test/libsolidity/syntaxTests/abiEncoder/invalid_pragma_value.sol

@@ -0,0 +1,3 @@
+pragma abicoder something;
+// ----
+// SyntaxError 2745: (0-26): Expected either "pragma abicoder v1" or "pragma abicoder v2".

test/libsolidity/syntaxTests/abiEncoder/same_setting_twice.sol

@@ -0,0 +1,4 @@
+pragma experimental ABIEncoderV2;
+pragma abicoder v2;
+// ----
+// SyntaxError 3845: (34-53): ABI coder has already been selected for this source unit.

test/libsolidity/syntaxTests/abiEncoder/select_v1.sol

@@ -0,0 +1 @@
+pragma abicoder v1;

test/libsolidity/syntaxTests/abiEncoder/selected_twice.sol

@@ -0,0 +1,4 @@
+pragma abicoder v1;
+pragma abicoder v1;
+// ----
+// SyntaxError 3845: (20-39): ABI coder has already been selected for this source unit.

test/libsolidity/syntaxTests/abiEncoder/selected_twice_v2.sol

@@ -0,0 +1,2 @@
+pragma abicoder v2;
+pragma experimental ABIEncoderV2;