[To-do] Drop fast_float

[tearfur]

What is the issue?

In a distant future where our minimum supported GCC version gets bumped to 11, we can drop the fast_float library :)

https://gcc.gnu.org/gcc-11/changes.html

Improved C++17 support, including:

• std::from_chars and std::to_chars for floating-point types.

Related:

• consider using from_chars() string-to-number parsing #3091

Which application of Transmission?

None

Which version of Transmission?

No response

[nevack]

spoiler

I like how you are confident that we will all be alive for this 😄

[Coeur]

We'll have converted the codebase to Rust before it happens.