HLSL: 1) Implement lookahead buffers/stacks for token advance/recede, 2) use it for cast operation.

The grammar now accepts type casts, like "(int)x", but that
has to be disambiguated from "(a + b)", needed deeper lookahead
and backing up than what existed so far.

Test/hlsl.cast.frag

@@ -0,0 +1,4 @@
+float4 PixelShaderFunction(float4 input) : COLOR0
+{
+    return (float4)input + (int4)input + (float4)1.198;
+}