Unify/abstract what it means to parse "a number of things" (#190)

* Unify parsing a number of things

Called `Length` for now, but open to other ideas!

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Author: stephencelis

Sources/Parsing/CountingRange.swift

@@ -0,0 +1,24 @@
+public protocol CountingRange {
+  var minimum: Int { get }
+  var maximum: Int? { get }
+}
+
+extension Int: CountingRange {
+  public var minimum: Int { self }
+  public var maximum: Int? { self }
+}
+
+extension ClosedRange: CountingRange where Bound == Int {
+  public var minimum: Int { self.lowerBound }
+  public var maximum: Int? { self.upperBound }
+}
+
+extension PartialRangeFrom: CountingRange where Bound == Int {
+  public var minimum: Int { self.lowerBound }
+  public var maximum: Int? { nil }
+}
+
+extension PartialRangeThrough: CountingRange where Bound == Int {
+  public var minimum: Int { 0 }
+  public var maximum: Int? { self.upperBound }
+}