Add compile-time support for reading/writing simple streams (fixes #77)

[nitely]

Fixes #77

Changes:

• Implements compile-time/vm support for memory inputs/outputs
• Changes writeText for floats so the output is the same across nim targets and versions. However, for float32 the output should now be the same as for float64. Nim +2.2.2 made this the default. The alternative is to keep the current behavior, but the result differs for the VM and across Nim versions, for example 1.23 is printed as 1.230000019073486 at runtime (sprintf) in Nim < 2.2.2, 1.23 in Nim >= 2.2.2, and 1.2300000190734863 (note the extra digit) at comptime (after cast[float32](cast[uint32](val)) the float32 val).

I checked json and cbor serializers passes most tests at comptime with these changes, the few that fail are unrelated to faststreams.

Tested for regressions here:

• bump nim-faststreams nimbus-eth2#7474
• fork; test faststreams comptime nitely/nim-json-serialization#1

[arnetheduck]

why not leave it as an overload?

[nitely]

The overload doesn't work in Nim 2.0.16 due to a Nim regression nim-lang/Nim#25166

[arnetheduck]

welcome to nim development ;) we note bugs in the code with # TODO https://github.com/nim-lang/Nim/issues/25166 so we can track them down later, at the point where they're relevant

[arnetheduck]

float roundtrip format lgtm - indeed, better to have it consistent and above all, round-trippable for the intended use case.

[arnetheduck]

hm, also, can replace the custom code in toml-serialization now?