Improve performance of VariableOrigin instances

- The previous approach would create a std::unique_ptr and store it in
  a std::list in VariableValue (Origins)
- The new approach now stores Origins in a std::vector and constructs
  VariableOrigin elements in-place on insertion.
- Instead of having two heap-allocations for every added VariableOrigin
  instance, this performs only one.
- If multiple origins are added, std::vector's growth strategy may even
  prevent a heap-allocation. There's a cost on growing the size of the
  vector, because a copy of current elements will be necessary.
  - Introduced reserveOrigin method to notify that multiple insertions
    will be made, so that we can use std::vector's reserve and do a
    single allocation (and copy of previous elements), and then just
    initialize the new elements in-place.

headers/modsecurity/variable_origin.h

@@ -15,6 +15,7 @@
 
 #ifdef __cplusplus
 #include <string>
+#include <memory>
 #endif
 
 #ifndef HEADERS_MODSECURITY_VARIABLE_ORIGIN_H_
@@ -36,14 +37,17 @@ class VariableOrigin {
     VariableOrigin()
         : m_length(0),
         m_offset(0) { }
+    VariableOrigin(size_t length, size_t offset)
+        : m_length(length),
+        m_offset(offset) { } 
 
-    std::string toText() {
-        std::string offset = std::to_string(m_offset);
-        std::string len = std::to_string(m_length);
+    std::string toText() const {
+        const auto offset = std::to_string(m_offset);
+        const auto len = std::to_string(m_length);
         return "v" + offset + "," + len;
     }
 
-    int m_length;
+    size_t m_length;
     size_t m_offset;
 };