Add deduction guides required for older Clang

Author: vector-of-bool

include/bson/parse.hpp

@@ -559,6 +559,9 @@ struct integer {
     }
 };
 
+template <typename R>
+explicit integer(R const&) -> integer<R>;
+
 /**
  * @brief A parser combinator that attempts to parse each element in a document/array
  *

src/amongoc/nano/util.hpp

@@ -476,6 +476,9 @@ struct opt_fmap {
     }
 };
 
+template <typename F>
+explicit opt_fmap(F const&) -> opt_fmap<F>;
+
 /**
  * @brief Decay-copy an object.
  *

src/amongoc/wire/message.hpp

@@ -59,6 +59,9 @@ struct body_section {
     std::uint8_t const& kind() const noexcept { return kind_byte; }
 };
 
+template <typename B>
+explicit body_section(const B&) -> body_section<B>;
+
 // Common body section type:
 extern template struct body_section<bson_view>;
 using bson_view_body_section = body_section<bson_view>;