Adding flatfile.HierarchyReader which is closely modeled after edi.reader but with more abstraction with the use of RecReader. (#165)

And more thorough unit level tests (compared with more end-to-end-ish tests in edi.reader)

For comparison, edi.reader:

https://github.com/jf-tech/omniparser/blob/927d69a1960aa8268a4e5eb828c605abd013f36c/extensions/omniv21/fileformat/edi/reader.go

Author: jf-tech

extensions/omniv21/fileformat/flatfile/.snapshots/TestRecDoneRecNext-root-A-B,_B_recDone,_moves_to_C,_no_target

@@ -0,0 +1,23 @@
+{
+	"Stack": [
+		{
+			"RecDecl": "#root",
+			"RecNode": "#root",
+			"CurChild": 0,
+			"Occurred": 0
+		},
+		{
+			"RecDecl": "A",
+			"RecNode": "A",
+			"CurChild": 1,
+			"Occurred": 0
+		},
+		{
+			"RecDecl": "C",
+			"RecNode": null,
+			"CurChild": 0,
+			"Occurred": 0
+		}
+	],
+	"Target": null
+}

extensions/omniv21/fileformat/flatfile/.snapshots/TestRecDoneRecNext-root-A-C,_C_recDone,_C_over_max,_A_becomes_target

@@ -0,0 +1,17 @@
+{
+	"Stack": [
+		{
+			"RecDecl": "#root",
+			"RecNode": "#root",
+			"CurChild": 0,
+			"Occurred": 0
+		},
+		{
+			"RecDecl": "A",
+			"RecNode": "A",
+			"CurChild": 0,
+			"Occurred": 1
+		}
+	],
+	"Target": "A"
+}

extensions/omniv21/fileformat/flatfile/.snapshots/TestRecDoneRecNext-root-A-C,_C_recDone,_C_over_max,_A_becomes_target,_but_target_xpath_no_match

@@ -0,0 +1,17 @@
+{
+	"Stack": [
+		{
+			"RecDecl": "#root",
+			"RecNode": "#root",
+			"CurChild": 0,
+			"Occurred": 0
+		},
+		{
+			"RecDecl": "A",
+			"RecNode": null,
+			"CurChild": 0,
+			"Occurred": 1
+		}
+	],
+	"Target": null
+}

extensions/omniv21/fileformat/flatfile/.snapshots/TestRecDoneRecNext-root-A-C,_C_recDone,_stay,_no_target

@@ -0,0 +1,23 @@
+{
+	"Stack": [
+		{
+			"RecDecl": "#root",
+			"RecNode": "#root",
+			"CurChild": 0,
+			"Occurred": 0
+		},
+		{
+			"RecDecl": "A",
+			"RecNode": "A",
+			"CurChild": 1,
+			"Occurred": 0
+		},
+		{
+			"RecDecl": "C",
+			"RecNode": "C",
+			"CurChild": 0,
+			"Occurred": 1
+		}
+	],
+	"Target": null
+}

extensions/omniv21/fileformat/flatfile/.snapshots/TestRecDoneRecNext-root-D,_D_recDone

@@ -0,0 +1,11 @@
+{
+	"Stack": [
+		{
+			"RecDecl": "#root",
+			"RecNode": "#root",
+			"CurChild": 0,
+			"Occurred": 1
+		}
+	],
+	"Target": null
+}