PP: Implement token pasting for PP identifiers.

Implement token pasting as per the C++ specification, within the current
style of the PP code.
Non-identifiers (turning 12 ## 10 into the numeral 1210) is not yet covered;
they should be a simple incremental change built on this one.
Addresses issue #255.

gtests/AST.FromFile.cpp

@@ -178,6 +178,7 @@ INSTANTIATE_TEST_CASE_P(
         "syntaxError.frag",
         "test.frag",
         "texture.frag",
+        "tokenPaste.vert",
         "types.frag",
         "uniformArray.frag",
         "variableArrayIndex.frag",