program-entrypoint: Add no_std attribute

[sonicfromnewyoke]

Problem

Currently the program-entrypoint crate is no_std "friendly" but does not have the crate attribute and used reexports from core and alloc in std

Solution

• Add the #![no_std] attribute to the crate.
• use direct import from core and alloc instead of reexports from std

[febo]

Looks good – just one nit.

[febo]

Looks good, thanks!

[LucasSte]

Adding no_std to entry point means that a #[panic_handler] must be implemented for the crate, since panics won't be processed in the STD;

[febo]

Adding no_std to entry point means that a #[panic_handler] must be implemented for the crate, since panics won't be processed in the STD;

Good point! When the custom_panic_default is used, it is not a problem since that macro brings the std – e.g., it uses msg!. But we should consider the case for when it is not used. We could either add a nostd_panic_default macro, or we need to document this case so when the program using the macro is no_std, it will need to set one up.

cc: @joncinque

Need to consider the potential issue with the panic handler.