`/dev/sgx_enclave` fixed mappings

[jarkkojs]

I created this PR for Enarx, which replaces mmarinus with vm-memory in every possible location except SGX fixed mappings:

enarx/enarx#2617

I.e., after:

$ git grep mmarinus
Cargo.lock: "mmarinus",
Cargo.lock:name = "mmarinus"
Cargo.toml:mmarinus = { workspace = true }
Cargo.toml:mmarinus = { version = "0.4.0", default-features = false }
src/backend/sgx/builder.rs:use mmarinus::{perms, Map, Shared};
src/backend/sgx/enarxcall.rs:use mmarinus::{perms, Map, Shared};
src/backend/sgx/mod.rs:use mmarinus::{perms, Map};

These remain essentially because I did not find API route to do fixed mappings for /dev/sgx_enclave. I partly created the PR to demonstrate this bottleneck the crate when scaling for SGX enclaves (partly because it would probably make sense for Enarx to use a popular mm crate).