compiler: Add Preprocessor Support

TODO: Test that file names are properly tracked in included
files.

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>

TestData/Sources/nested-split-2.p4

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+#include <nested-split-a.p4>
+#include <nested-split-b.p4>

TestData/Sources/nested-split-a.p4

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+       state start {

TestData/Sources/nested-split-b.p4

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+           transition select (false) {
+              true: reject;
+              false: reject;
+           };
+       }

TestData/Sources/nested-split.p4

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+    parser main_parser() {
+#include <nested-split-2.p4>
+    };

TestData/Sources/simple-split-oneline.p4

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+#include <testing-split-oneline-a.p4>#include <testing-split-oneline-b.p4>

TestData/Sources/simple-split.p4

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+#include <testing-split-a.p4>

TestData/Sources/simple-unfound.p4

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+#include <unfound.p4>

TestData/Sources/simple.p4

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+#include <testing.p4>

TestData/Sources/testing-split-a.p4

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+    parser main_parser() {
+       state start {
+#include <testing-split-b.p4>

TestData/Sources/testing-split-b.p4

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+           transition select (false) {
+              true: reject;
+              false: reject;
+           };
+       }
+    };

TestData/Sources/testing-split-oneline-a.p4

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+    parser main_parser() {
+       state start {
+           transition 

TestData/Sources/testing-split-oneline-b.p4

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+select (false) {
+              true: reject;
+              false: reject;
+           };
+       }
+    };

TestData/Sources/testing.p4

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+    parser main_parser() {
+       state start {
+           transition select (false) {
+              true: reject;
+              false: reject;
+           };
+       }
+    };