Improve allocators set up

[febo]

Problem

Currently, the "fallback" panic handler and allocator set up creates different problems when using cargo test or cargo test-sbf. This happens since tests in general depend on the std and therefore create a conflict when the fallback tries to set up a panic handler or global allocator.

Solution

This PR improves the set up of the fallbacks by simply linking the std library instead of trying to set them up, avoiding conflicts if another library links the std. Note that the std is only linked in targets different than "solana", so no_std programs are not affected.

It also addresses a couple of comments from #136:

• deprecate entrypoint macro in favour of entrypoint_with_allocator_and_panic_handler
• deprecate no_allocator macro in favour of static_allocator
• remove the need for feature(const_mut_refs)

cc: @publicqi

[joncinque]

Looks great! I'm a big fan of the lengthy and clear comments.

Just a tiny nit

[febo]

The refactoring for the static_allocator will be added to a separate crate.