Support customizing how floats are serialized

[jwodder]

Add options for configuring the following aspects of how float values are serialized to JSON:

• How many digits to show after the decimal point
• Whether to remove trailing zeroes after the decimal point
• Whether to use %e or %f format
• %g format: Use %f if exponent is within a given range, otherwise use %e
• Whether to place a + after an e?
• Whether to use e or E?

[jwodder]

It seems that the Ryū Printf algorithm described at https://dl.acm.org/doi/10.1145/3360595 and implemented in C++ at https://github.com/ulfjack/ryu can be used to produce one's choice of %e- or %f-style floating point strings, in contrast to the original Ryū algorithm used by ryu (used in turn by serde_json), which always outputs the shorter of the two formats for a given number. However, there do not appear to be any Rust implementations of the Ryū Printf algorithm yet, and the maintainer of ryu does not appear to be interested in adding such functionality.