Portable float NIF

[deadtrickster]

Replace #62 with more portable nif (courtesy of @benoitc)
(Extracted from https://gitlab.com/barrel-db/instrument, Gauge renamed to Value, instrument namespace to prometheus and target renamed from instrument_nif to prometheus_nif)

TODO

• Limit number of iterations to prevent long-running or endless? CAS loops
• Implement prometheus_value which tries to load this nif but fallbacks to gen_server (Windows)
• Make compilation conditional via envvar (we don't need nif troubles with rabbitmq plugin-like use cases, where we don't instrument directly)
• Adjust makefile targets + finally add rebar3_archive_plugin

Thanks @benoitc ;-)

[deadtrickster]

default travis vms are very old (