add information and minor restructuring

src/case.md

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+# Case Statements
+
+Nimrod also supports case statements, which are like switches in other languages. There are several things to note here:
+
+* You can use strings in the switch statement
+* Sets and ranges of ordinal types are also usable
+* case statements, like most things, are actually expressions
+* It is required that every possible case be covered
+
+``` Nimrod
+case "charlie"
+of "alfa":
+  echo "A"
+of "bravo":
+  echo "B"
+of "charlie":
+  echo "C"
+else:
+  echo "Unrecognized letter"
+
+case 'h':
+of 'a', 'e', 'i', 'o', 'u':
+  echo "Vowel"
+of '\127'..'\255':
+  echo "Maybe, but I only know English"
+else:
+  echo "Consonant"
+
+proc positiveOrNegative(num: int): string =
+  result = case num
+  of low(int)..-1:
+    "negative"
+  of 0:
+    "zero"
+  of 1..high(int):
+    "positive"
+  else:
+    "impossible"
+
+echo positiveOrNegative(-1)
+```
+
+```
+$ nimrod c -r ./case_stmts.nim
+C
+Consonant
+negative
+```