Make f128 crate and struct be a command line option

[LegNeato]

As it is, code with long double includes an unmaintained crate that only works on Linux (and may even be broken there): f128. It looks like rust_decimal with the c_repr feature could be a potential path forward.

Related to #154.

[kkysen]

A few points:

• f128 is not unmaintained; the README.md says "This crate is currently in maintenance mode.".

• long double in C can map to very different things. msvc uses double for long double, for example (though we don't support Windows at all at this point). So trying to have good cross-platform behavior with long double is really hard, and c2rust transpile doesn't have a good cross-platform story in the first place due to the preprocessor.

• I'm confused how rust_decimal could help, though. It looks like it's for a 96-bit decimal representation, while long double is often f128 aka IEEE754 binary128. Am I misreading things?

• Given f128 is not a community standard at all, and until there is an f128 or long double (c_long_double) in std/core, we could also make what crate and struct gets used for f128 support be a command-line option.

[LegNeato]

No, you are correct, I think this is invalid (got confused by "The corresponding target layout is 128 bit aligned" in the rust_decimal docs) and a command option is the correct path.