Implement type conversions on BigNum

[npmccallum]

The type conversions both closely wrap existing (1.1.0) OpenSSL APIs and are idiomatic Rust.

Deprecate: BigNum::to_vec(), BigNum::from_slice() and BigNum::from_u32() in favor of the new idiomatic APIs.

Also, silence warnings about std::mem::uninitialized() on newer Rust.

Minimum Required Rust Version is now 1.34 (for TryFrom).

[npmccallum]

This PR replaces #1203 and #1204.

[npmccallum]

@sfackler ping

[npmccallum]

@sfackler bump

[npmccallum]

@sfackler We are patching for this downstream. The latest release broke our build. Could you please review this so that we can get it in a release soon? Thanks!

[npmccallum]

@sfackler Releasing v0.10.28 broke our build again. Can you please review this PR?

[npmccallum]

@sfackler Could you at least comment and let me know the status of this PR? Thanks!

[npmccallum]

@sfackler Any update? v0.10.29 broke our build again.

[npmccallum]

@sfackler I've rebased this PR against master.

Today marks 8 months since I first submitted this PR; but I haven't heard anything from you. You still seem to be active in pushing releases, so I'm assuming you're around. I'm happy to trade reviews for something that is a high priority for you (either here or elsewhere...) if you'd like. Please let me know anything I can do to help.

[petreeftime]

LGTM! Makes conversions a lot more idiomatic, but I think this would need a version bump from 0.10 to 0.11, since it changes the interface quite a bit.

[npmccallum]

@petreeftime Although it does change the interface, this patch set is fully backwards compatible. A bump from 0.10 to 0.11 indicates a break in API/ABI which this patch doesn't implement. Do you still want a version bump?

[petreeftime]

I think deprecated functions can break builds for people in some situations, so it might be safer to have a version bump, which would prevent this. I am not an owner of this repo, so ultimately it's not my decision, I just like the patches and wanted to implement something similar.