Speed up number serialization using itoa and ryu crates

[Marcono1234]

Problem solved by the enhancement

The speed of number serialization could probably be improved.

Enhancement description

Investigate usage of the libraries https://docs.rs/itoa and https://docs.rs/ryu for number serialization (see also TODO comments in Struson code). Have to verify that using them increases the serialization speed.

Usage of these crates should be controlled by a (single) opt-in feature. This keeps the number of dependencies for the default features low and might avoid potential incompatibilities between the serialized output.