Replace std for alloc

[febo]

Problem

Currently, pinocchio offers a "std" feature to enable code that requires memory allocation using the std library, although the same functionality is available through alloc.

Solution

Replace the "std" feature by "alloc" and use the alloc library instead.

The "alloc" feature is enabled by default and can be disabled by adding the dependency with default-features = false.

[Aursen]

The main usage of the old std was for the alloc one (all other usages are available in the core). I think it's a good point to force people to build an sdk outside of the program itself

[ilya-bobyr]

Maybe someone else needs to look at this change, but it looks good to me.
If it compiles, it probably is correct :)
If you want, I can approve it.