Add lexing, parsing and some semantic checking for constant buffers.

test/constant_buffer.shd

@@ -0,0 +1,12 @@
+Camera_Data :: constant_buffer {
+	projection : float4x4;
+	view       : float4x4;
+}
+
+vertex main :: (pos : float4 @position) -> float4 @position {
+	return mul(projection, mul(view, pos));
+}
+
+pixel main :: () -> float4 @target {
+	return float(0.5, 0.5, 0.5, 1.0);
+}