Experimental standard library

Change import syntax and cover with tests

test/libsolidity/syntaxTests/experimental/parsing_stdlib_import_1.sol

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+pragma experimental solidity;
+
+import std.stub;
+// ====
+// EVMVersion: >=constantinople
+// ----
+// Warning 2264: (std.stub:63-92): Experimental features are turned on. Do not use experimental features on live deployments.
+// Warning 2264: (0-29): Experimental features are turned on. Do not use experimental features on live deployments.

test/libsolidity/syntaxTests/experimental/parsing_stdlib_import_2.sol

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+pragma experimental solidity;
+
+import std.stub as stub;
+// ====
+// EVMVersion: >=constantinople
+// ----
+// Warning 2264: (std.stub:63-92): Experimental features are turned on. Do not use experimental features on live deployments.
+// Warning 2264: (0-29): Experimental features are turned on. Do not use experimental features on live deployments.

test/libsolidity/syntaxTests/experimental/parsing_stdlib_import_3.sol

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+pragma experimental solidity;
+
+import { identity } from std.stub;
+// ====
+// EVMVersion: >=constantinople
+// ----
+// Warning 2264: (std.stub:63-92): Experimental features are turned on. Do not use experimental features on live deployments.
+// Warning 2264: (0-29): Experimental features are turned on. Do not use experimental features on live deployments.

test/libsolidity/syntaxTests/experimental/parsing_stdlib_import_4.sol

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+pragma experimental solidity;
+
+import * as stub from std.stub;
+// ====
+// EVMVersion: >=constantinople
+// ----
+// Warning 2264: (std.stub:63-92): Experimental features are turned on. Do not use experimental features on live deployments.
+// Warning 2264: (0-29): Experimental features are turned on. Do not use experimental features on live deployments.

test/libsolidity/syntaxTests/experimental/parsing_stdlib_import_without_pragma_1.sol

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+import std.stub;
+// ----
+// ParserError 9478: (7-10): Expected string literal (path), "*" or alias list.

test/libsolidity/syntaxTests/experimental/parsing_stdlib_import_without_pragma_2.sol

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+import { identity } from std.stub;
+// ----
+// ParserError 6845: (25-28): Expected import path.