Speeding up tippecanoe-overzoom

[e-n-f]

This PR consists of many, mostly tiny, optimizations to tile reading and writing to improve the speed of tippecanoe-overzoom. There should be no externally visible impact other than greater speed. The specific changes are:

attribute

• change from map to unordered_map for attribute accumulation

clip

• only convert the attributes that have an --accumulate-attribute set from mvt_value to serial_val and back; copy the others directly into the output as mvt_value.
• make a couple of strings constant instead of indulging in runtime conversion of C strings to C++ strings
• avoid a round trip through serial_val when reading the tippecanoe:retain_points_multiplier_sequence
• don't bother clipping and cleaning geometries if the source and destination tile are the same and the tile extent is not being reduced
• add a couple of std::move to avoid copying data

decode

• fix a pmtiles warning about a potentially uninitialized variable

evaluator

• be more careful to use const references instead of value arguments
• distinguish mvt_value types with a switch instead of a series of ifs
• use a getter function instead of a map to pass the attributes around
  • add an mvt_no_such_key type to distinguish does-not-exist from null, since old Mapbox-style expressions care, even though Felt expressions don't
  • in the mvt_feature attribute getter, do a linear scan for attributes instead of setting up a map, since most of the attributes will never be referenced anyway

main

• more rippling changes from map to unordered_map

mvt

This one has a more substantive change. The mvt_value structure previously always contained a std::string for the string value case. Now it has a shared_ptr<std::string>, which may be null in the non-string cases. The string that is pointed to may be unique to the mvt_value, or it may be shared across an entire tile, so that the text of string values is concatenated into the shared pool string instead of requiring a separate string allocation in each mvt_value.

The specific string value for each mvt_value is referenced as an off and len into the pool string. When possible, callers access the string value as a string_view or c_str instead of through a substring copy.

In addition:

• In mvt decoding, use reserve, std::move, and emplace_back to avoid repeated construction and copies
• decoding does not create the map from keys and values to their unique IDs, since this is only really useful when writing tiles, not reading them
• decoding makes a shared pool string for its string attribute values, to avoid repeated allocations in each decoded value
• switches instead of ifs
• a new getter function to get numeric values out of mvt_value without round-tripping through serial_val
• a hash function for mvt_value so it can be used in unordered_map

overzoom

• change from map to unordered_map

pmtiles

• fix a couple more warnings

serial

• change from map to unordered_map

tile-join

• fix a warning
• change from map to unordered_map

tile

• unordered_map
• use new string value getters and setters

write_json

• if to switch