Add missing const in function signatures

[mpg]

Add missing const in function signatures. This issue is only about simple cases where the const was just forgotten and no code change is needed in order to add it.

See comments below for an incremental list.

[mpg]

If still public: mbedtls_ecp_write_key()'s first parameter - it should always have been const, probably an oversight.

[gilles-peskine-arm]

From what I remember from the discussions leading to 3.0, a lot of functions operating on ECP objects have to use a non-const pointer due to the lazy loading of group data, which means that mbedtls_ecp_group members have to be non-const in many low-level functions, which then infects the whole interface with non-constness.

Previous discussion: #1485

[mpg]

Yes, but that's only for functions that may end up callling mbedtls_ecp_mul(). That's not the case here. (From reading the code, and also trying to add a const and see if the compiler complains - it does not.)

[mpg]

Btw, I just checked, and mbedtls_ecp_mul() is still not quite ready yet to have its first argument const, though that would only be a moderate amount of work now that the pre-computed multiples table are stored as static data, instead of computed at runtime. The trouble is, the code still supports computing it at runtime, which is only useful when adding a new curve, as we compute the table at runtime then dump it before we add it to ecp_curves.c, and then it never needs to be computed at runtime again - see scripts/ecp_comb_table.py.

We should really get rid of that and instead compute the table for new curves in python or using a dedicated C program, but it's ridiculous to keep code in all builds of the library that's only useful when developing the library, not when using it. Especially when it shows in the API, as some parameters that should morally be const are not.

So, I'm not adding ecp_mul() & Co here. This issue is only for const qualifiers that can just be added because they were just forgotten.

[mpg]

Created #7601 to track preliminary work that's needed before making ecp_mul() const.