Reduce tippecanoe memory usage (#5)

* Change JSON objects to a union type to use less memory

* Stop storing the string representation of JSON numbers

* Restore the ability to create features with large integer attributes

* Make sure large-integer feature IDs still behave as before

* Add missing #include

* Don't preallocate as much space for arrays and objects

* Treat inability to check free disk space as a warning, not an error

* Update changelog and version

milo/milo.h

@@ -0,0 +1,9 @@
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+char *dtoa_milo(double val);
+
+#ifdef __cplusplus
+}
+#endif