Remove dependency on micromath, compute height with integers only

[matthiasgoergens]

This reduces the overall dependency tree, which was only being used in computing the tree height.
It may also be benefitial in some environments (e.g.: zk) to be able to use integer math only.

The snippet leaves_count as f32 caused some trouble. Let's remove it.

(Description stolen from #16 because it's honestly much better than what I had originally typed up.)

[sashaduke]

To expand on this - the use of floats is not supported on CosmWasm's WASM VM, so removing this means the crate will now be able to used in smart contracts deployed on CosmWasm without any issues.

[matthiasgoergens]

To expand on this - the use of floats is not supported in WebAssembly, so removing this means the crate will now be able to used in programs which are compiled to WASM without any issues.

To be more precise WebAssembly supports floats just fine by itself, but Sasha has an application that only supports a subset of WASM that doesn't support floats.

[antouhou]

@matthiasgoergens @sashaduke This uses std though, which is not compatible with the no-std feature. I'll try to think how to do that without using an f32 and std, thank you for reporting this!

[matthiasgoergens]

Thanks for checking!

I wonder if we can ask the compiler or so to enforce the no-std rule?

[sashaduke]

no_std compatibility can be maintained by using core::mem::sizeof instead of std::mem::sizeof

[matthiasgoergens]

@sashaduke Thanks, I implemented your suggestion!

@antouhou I can build the fixed version with cargo build --no-default-features (and just to confirm, this check fails when trying on my original version).

I think about how to automate this check.