fix: use core::mem instead of std::mem in no_std zkvm precompile module

[sergey-melnychuk]

The zkvm module in src/algorithms/rsa.rs is compiled only for cfg(all(target_os = "zkvm", target_vendor = "succinct")), which is a no_std target. The module used std::mem::transmute and std::mem::size_of, causing build failures when the std feature is disabled (the typical setup for SP1 guest programs).

Replace all five occurrences with their core::mem equivalents, which are always available regardless of std.

Error before this fix

error[E0433]: failed to resolve: use of unresolved module or unlinked crate std
--> src/algorithms/rsa.rs:101:21
|
| std::mem::transmute::<[Chunk; $chunks * 2], [u8; $bytes * 2]>(prod_chunks)
| ^^^ use of unresolved module or unlinked crate std

Reproduction

Add rsa = { version = "0.9", default-features = false } to an SP1 guest program and run cargo prove build.

[sergey-melnychuk]

Note: This branch is based on tag patch-0.9.6-sp1-6.2.0, not master. The extra commits in the diff are from that tag's patch series — the actual change is a single commit touching 5 lines in src/algorithms/rsa.rs.

If this should be rebased onto a different base branch, please let me know and I'll update it.