Render constrained system friendly

[ithinuel]

This is to allow building and running wasmi on constrained target such as microcontroller.
This requires the ability to build without std.

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[pepyakin]

I am not sure what is this PR all about. Can you elaborate a bit?

[ithinuel]

@pepyakin Thanks for your review.
This is to allow building and running wasmi on constrained target such as microcontroller.
This requires the ability to build without std.
I updated the PR's description.

Since your first review I updated the truncation related changes. I removed the dependency on BigRational. It does not break the tests and allows to build with nostd.

[ithinuel]

This work is also related to paritytech/parity-wasm#283

[ithinuel]

Hi there, sorry for the way too long delay since my last update.
I ran some tests and removed the changes that are no longer required (around the float primitives).

[pepyakin]

Thanks, @ithinuel !