Improve fallback for no_std

[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 – e.g., they will continue to be no_std.

It also addresses a comment from #136, removing the need for feature(const_mut_refs) to use allocate_unchecked.

cc: @publicqi

[sonicfromnewyoke]

tested locally, have no annoying errors

[joncinque]

This looks good to me!

I was worried about a case where someone wants to use a program crate as a dependency in a nostd environment that isn't Solana, then they would get std without knowing better.

This situation shouldn't happen though, since program entrypoints should only be enabled when building a Solana program, so I think we're all good!