Speeding up tippecanoe-overzoom (#191)

* Speed up mvt_value comparison

* Converting repetitive ifs to cases

* More conversions from ifs to cases

* Optimize the always-true filter case

* Don't convert types of attributes without accumulators

* Unordered map seems to be faster than map

* Add missing header

* Fix some warnings

* Fix the warnings better

* Avoid an int->string->int conversion

* Lazily initialize layer key and values maps when actually needed

* More switches from maps to unordered_maps

* Sure, I'll take the microoptimization

* More emplacement

* Save some copies

* Emplaces and moves

* Lazy linear scan of attributes instead of building a map

* Extra printfs, missing header

* Avoid clipping if the input and output tiles are the same

* But do clip if the tile extent is being reduced

* Make sure I'm not constructing std::strings here at runtime

* More worrying about runtime string construction

* A couple more std::moves

* Const references!

* More const references

* Another std::move

* Make the string_value of mvt_value std::optional

* Reserve storage when decoding

* Provision for different mvt_values to share a string pool

* Use the string pool when decoding

* Avoid another string construction

* Try limiting the depth of the search for duplicate attributes

* Revert "Try limiting the depth of the search for duplicate attributes"

This reverts commit 9ec94a15ff.

* Update changelog

* Fix typo noticed during code review

serial.cpp

@@ -878,7 +878,7 @@ int serialize_feature(struct serialization_state *sst, serial_feature &sf) {
 	return 1;
 }
 
-void coerce_value(std::string const &key, int &vt, std::string &val, std::map<std::string, int> const *attribute_types) {
+void coerce_value(std::string const &key, int &vt, std::string &val, std::unordered_map<std::string, int> const *attribute_types) {
 	auto a = (*attribute_types).find(key);
 	if (a != attribute_types->end()) {
 		if (a->second == mvt_string) {