Updated Lexer::make_lexer to return a unique_ptr

This is part of my quest to replace all unneeded std::shared_ptrs for
std::unique_ptrs to improve performance, since there is no need for
reference counting for pointers that are uniquely used. I also had to
change the first parameter of Lexer::assign_parent_location to a
std::unique_ptr<Lexer>& so that it can accept the new result of
make_lexer.

src/Lexer.cpp

@@ -60,12 +60,12 @@ std::string Lexer::recalculate_current_line(const std::string& text)
     return final_str;
 }
 
-std::shared_ptr<Lexer> Lexer::make_lexer(const std::string& fname)
+std::unique_ptr<Lexer> Lexer::make_lexer(const std::string& fname)
 {
-    return std::shared_ptr<Lexer>(new Lexer(fname)); // not using make_shared because the constructor is private
+    return std::unique_ptr<Lexer>(new Lexer(fname)); // not using make_shared because the constructor is private
 }
 
-void Lexer::assign_parent_location(std::shared_ptr<Lexer>& lexer, const std::shared_ptr<Location>& loc)
+void Lexer::assign_parent_location(std::unique_ptr<Lexer>& lexer, const std::shared_ptr<Location>& loc)
 {
     lexer->loc.parent = loc;
 }

src/Lexer.h

@@ -46,8 +46,8 @@ class Lexer
     TokenStream lex(const std::string& text);
 
     /* Create a new Lexer and return a pointer to it. */
-    static std::shared_ptr<Lexer> make_lexer(const std::string& fname);
+    static std::unique_ptr<Lexer> make_lexer(const std::string& fname);
 
     /* If the Lexer is lexing an impòrted file, give it the location in the parent file at which it was imported. */
-    static void assign_parent_location(std::shared_ptr<Lexer>& lexer, const std::shared_ptr<Location>& loc);
+    static void assign_parent_location(std::unique_ptr<Lexer>& lexer, const std::shared_ptr<Location>& loc);
 };