Simplify native reading (#54)

* Simplify native reading

Simplify the operations of reading and tokenizing an input source file that is either Boot or Spad.

* Fix thinko

* Use `std::numeric_limits` to express intent

src/include/open-axiom/InputFragment

@@ -1,5 +1,5 @@
 // -*- C++ -*-
-// Copyright (C) 2014-2015, Gabriel Dos Reis.
+// Copyright (C) 2014-2024, Gabriel Dos Reis.
 // All rights reserved.
 // Written by Gabriel Dos Reis.
 //
@@ -127,6 +127,16 @@ namespace OpenAxiom {
    };
 
    std::ostream& operator<<(std::ostream&, const Fragment&);
+
+   // A prose is the contents of an input source file organized as a
+   // sequence of fragments.
+   // Note: a prose is defined as movable, but not a copyable type.
+   struct Prose : std::vector<Fragment> {
+      Prose() = default;
+      Prose(Prose&&) = default;
+   };
+
+   Prose read_source(std::istream&);
 }
 
 #endif  // OPENAXIOM_INPUTFRAGMENT_included