Remove hashbrown and use BTree{Map,Set} from the alloc crate

[ghost]

wasmi-validation must handle untrusted input and hence we switch from
Hash{Set,Map} (whether std's or hashbrown's) to BTree{Set,Map} to avoid
algorithmic complexity attacks while retaining no_std support.

Closes #183

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[ghost]

@pepyakin I added an additional commit remove the *Set from wasmi-validation entirely, as we should get the same O(N log N) worst-case behaviour using a sorted slice that makes due with a single allocation and should yield better memory locality than the BTreeSet.

[pepyakin]

Thanks!

[goral09]

Recent version of Rust 1.36.0 started using hashbrown::HashMap implementation in the standard library. Just saying.

[ghost]

Recent version of Rust 1.36.0 started using hashbrown::HashMap implementation in the standard library. Just saying.

This was about replacing Hash Brown by a different hash map implementation (as it was for the standard library), but rather by replacing a hash map which is susceptible to collision complexity attacks by a BTree-based implemenation which has a much simpler worst-case complexity behavior and also has no_std support (which the standard libraries's Hash Brown implementation lacks for now). (Also the standard library uses SipHash as the default hasher instead of FxHash as Hash Brown exactly to harden itself against these complexity attacks.)