PoC: Add hazmat module (exposing scalar, point)

[theStack]

This PR adds a "hazmat" [1] module for accessing internal data structures and functions (scalar and point for now), with the idea that other libraries could build on top of that. The implementation is inspired by secp256k1-zkp/#153, but aims to provide data types that can be allocated on the stack instead of the heap, as it seems to be significantly simpler from a user perspective if the burden of dynamic memory management can be avoided. The approach to achieve this is by using a "shell type" with fixed size and alignment that hosts the underlying structure not visible to the user [2], eg for scalar:

typedef union {
    unsigned char data[32];
    uint64_t align8; /* ensure alignment on 8-bytes boundaries */
} secp256k1_hazmat_scalar;

Without the alignment enforcing dummy, the compiler emits warnings like "warning: cast from 'const secp256k1_hazmat_scalar *' to 'secp256k1_scalar *' increases required alignment from 1 to 8 [-Wcast-align]" when the public type is converted to the underlying internal type via casting. This admittedly looks a bit fishy and I'm not sure if this is even standard-compliant C89, so would appreciate feedback particularly on that.

For exposing the ge/gej types, a "smart" point object is used that stores the result in jacobi coordinates together with a "z-is-one" flag and converts to affine coordinates when needed (which is trivial by simply assigning x/y if that flag is set). For multiplication with the generator point, a secp256k1 context object is needed, as otherwise it wouldn't be possible to take use of the optimized and more secure variant.

See the added example for how this module could be used in practice.

For further open questions, see the earlier writeup https://gist.github.com/theStack/0893258c3aba91c28539b9334d2e0608.

[1] ="hazardous material"; this terminology origins in Python's pyca/cryptography library, see https://cryptography.io/en/latest/hazmat/primitives/ ("You should ONLY use it if you’re 100% absolutely sure that you know what you’re doing because this module is full of land mines, dragons, and dinosaurs with laser guns.")
[2] see e.g. https://fastcompression.blogspot.com/2019/01/opaque-types-and-static-allocation.html