Add alloc feature to the crate so that we can port it to no_std

[CPerezz]

Currently, migrating the crate to no_std without the usage of allocators is "possible" but completely useless.

We need to use Vec-like structures, and the only full no_std compatible libs that don't require alloc to work are arrayvec and const-arrayvec. Both of them will eat the entire stack of any regular machine if we try to compute big Proofs (2^16 or more constraints).

Therefore, our best bet is to use an alloc gate for the time being, and if we're in enviroments such as WASM where we want to generate proofs, there's some work already done in allocators there. For embedded I think it's quite impossible to do so if the proofs are bigger than a thousand constraints, maybe much less.

So the idea is to port the repo to no_std and move all of the Vec related work behind an alloc feature. On that way, at least the data structures such as Proof or others can be fully no_std compatible even an allocator will be needed for sure.

[CPerezz]

Closed via #495