[Feature]: support `no_std` environments

[akubera]

This changes the minimum supported rust version to 1.43, by my estimation. Though I don't really like we're straying away from num-bigint's 1.31, I don't think too many people will be upset by dropping versions old than 3 years.

Let's update version in the README (last line)

[bigherc18]

@akubera

Let's update version in the README (last line)

I can't see any version mentioned in the README

[bigherc18]

Edit: Fixed

This changes the minimum supported rust version to 1.43

We can also avoid this easily, because it comes from using siphasherand it's only used during tests, we should either replace the siphasher with a crate that support edition = 2015 or only run the tests that uses hash when on std

What do you think ?

[bigherc18]

@akubera would you please take a look again, we no longer need to change the minimum supported version

We're using your bigdecimal-rs library in our projects and we need the no_std support, we'd like you to merge this PR instead of maintaining our own fork

[eranrund]

We would also love to see this gets merged! Any chance that might happen?

[tdelabro]

@akubera What is preventing us to merge?

[akubera]

I think we have to require alloc so I'm removing that optional feature.

Unless I'm missing something, num-bigint requires alloc, so we're not restricting our environments any more than our dependencies.