Improve version management & avoid conflict between multiple version

[Taowyoo]

Purpose

To make it's possible to have multiple versions of rust-mbedtls in the dependency tree.

The following works need to be done:

Able to compile together:

• Add namespace for mbedtls_malloc to avoid conflicts. Namespace symbols for mbedtls_malloc to avoid conflicts. #243
• Move self test logic (including exclusive C function definitions and exported rust functions) to a separate crate Move self test logic to a separate crate #244
• Move all platform specific exported function ( threading time selftest ) into a separate crate Add crate: mbedtls-platform-support #254

Better version management:

• Publish first version of mbedtls-selftest, done by Prepare to publish mbedtls-selftest #250
• Back-port all above changes to https://github.com/fortanix/rust-mbedtls/tree/v0.7, done by [v0.7] Move self test logic to crate mbedtls-selftest #251
• publish new version 0.7.3
• publish new version 0.8.2
• back-port Add crate: mbedtls-platform-support #254 to v0.7, [v0.7] Move selftest, threading, time code to crate mbedtls-platform-support #260
• publish new version 0.7.4
• back-port Add crate: mbedtls-platform-support #254 to v0.8, [v0.8] Move self test, threading, time code to crate mbedtls-platform-support #259
• publish new version 0.8.3

[Taowyoo]

Just realize that there also a lot of rust functions and function pointers are exposed in lib.rs and threading.rs.
They will also cause conflict, those functions could be solved by following same path of self test but the function pointers might be difficult to deal with.

[jethrogb]

Yeah I guess we could make another crate mbedtls-platform-support. Especially for lib.rs I'm not sure if that will work properly for 0.7, because the features & conditional build logic supporting this changed pretty significantly in 0.8. I don't think the function pointers are a big deal, should work the same way?

[jethrogb]

I'm not sure if that will work properly for 0.7

If it won't, then I think we should give up on this for now, possibly revert the mbedtls-selftest changes as well, and just do a flag day migration across all our projects.

[Taowyoo]

Got it, let me try if it's possible to split the code in 0.7