fast_double_parser

I also did this when searching for a fast string-to-float conversion function for ClickHouse: https://github.com/ClickHouse/ClickHouse/blob/master/src/IO/... - we create a table, insert all four billion floats there, and check our functions with SQL queries. Currently, the state-of-the-art library is https://github.com/lemire/fast_double_parser, but we use a slightly worse (but more performant) routine. It is also interesting how to format floats in string in an efficient and user-friendly way. For this problem, we switched iteratively: strtod -> Google's Double Conversion -> Ryu (patched) -> Dragonbox. Currently, we use Dragonbox.