hecl/hecl: Allow CaseInsensitiveCompare to be used with heterogenous lookup

It's quite common to see maps or sets that make use of a string->object
association, usually as <std::string, T>. However, this can result in
inefficient code when performing lookups or indexing, as something like:

std::map<std::string, T> some_map;

...

auto iter = some_map.find("Name");

...

will result in a std::string instance being constructed around "Name",
which is less than ideal.

With a heterogenous comparator, however (such as std::less<>), like:

std::map<std::string, T, std::less<>>

we allow the container to do automatic type deduction and comparisons.
This allows types comparable to the key type to be used in find calls,
etc, without actually needing to construct the key type around the other
type.

The main way to enable containers to perform such behavior is by
defining a type named is_transparent within the comparator type.

hecl/include/hecl/hecl.hpp

@@ -505,6 +505,9 @@ public:
  * @brief Case-insensitive comparator for std::map sorting
  */
 struct CaseInsensitiveCompare {
+  // Allow heterogenous lookup with maps that use this comparator.
+  using is_transparent = void;
+
   bool operator()(std::string_view lhs, std::string_view rhs) const {
     return std::lexicographical_compare(lhs.begin(), lhs.end(), rhs.begin(), rhs.end(), [](char lhs, char rhs) {
       return std::tolower(static_cast<unsigned char>(lhs)) < std::tolower(static_cast<unsigned char>(rhs));