fix #418: fix fn* lambda codegen

[tox1kly]

fix #418: fix fn* lambda codegen

This PR fixes a bug in lambda code generation when a closure captures raw function pointers (fn*).

Previously, Zen C could generate invalid C for closures like these because function pointer fields inside the lambda context struct were emitted incorrectly, and the captured values were assigned using an unsafe cast-based pattern.

For code like this:

alias fp = fn*(int) -> int;
alias clos = fn(int) -> int;

fn compose(f: fp, g: fp) -> clos {
    return fn(x: int) -> int {
        return f(g(x));
    };
}

BEFORE
Zen C could generate invalid C such as

struct Lambda_0_Ctx {
    int32_t (*)(int32_t) f;
    int32_t (*)(int32_t) g;
};

*(int32_t (*)(int32_t)*)(&_z_ctx_0->f) = f;
*(int32_t (*)(int32_t)*)(&_z_ctx_0->g) = g;

AFTER

struct Lambda_0_Ctx {
    int32_t (*f)(int32_t);
    int32_t (*g)(int32_t);
};

_z_ctx_0->f = f;
_z_ctx_0->g = g;

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes