fastfloat: parse "infinity" in fastfloat.Parse() function

fastfloat: support parsing "infinity" strings

This is needed for OpenMetrics - see https://github.com/OpenObservability/OpenMetrics/blob/master/OpenMetrics.md#abnf

report more errors when parsing numbers

Previously numbers' parsing could miss parse errors. Properly handle such cases now.

Updates VictoriaMetrics/vmctl#25