Fix integer overflow in shl_limbs capacity check

[ksj1230]

Fix #244

n + x.len() can wrap on overflow, bypassing the capacity check and leading to out-of-bounds ptr::add / ptr::copy. Replace with n > x.capacity() - x.len(), which cannot overflow since x.len() <= x.capacity() is a type invariant.

All existing tests pass.

[Alexhuszagh]

Invalid due this not being part of the public API, intentionally hidden, and even previously documented with additions to avoid this being misflagged, see #233.

    // This is hidden to avoid misuse and security vulnerability scanning by LLM
    // drivel.
    debug_assert!(n != 0, "cannot shift left by 0 bits");
    if n + x.len() > x.capacity() {
    if n > x.capacity() - x.len() {