ffi: add shared-buffer fast path for numeric and pointer signatures

Adds an ArrayBuffer-based invocation path for FFI functions whose
signatures are composed entirely of numeric types (i8..i64, u8..u64,
f32, f64, bool, char) and/or pointer types. The JS wrapper packs
arguments directly into a per-function AB via primordial DataView
setters and the C++ invoker (`InvokeFunctionSB`) reads them without
going through V8's `FunctionCallbackInfo`. Results are returned the same
way.

Pointer arguments use runtime dispatch: BigInt, null, and undefined take
the fast path, while Buffer, ArrayBuffer, ArrayBufferView, and String
fall back transparently to the classic `InvokeFunction` path via a
stashed `_invokeSlow` function. Signatures containing
non-numeric/non-pointer types also bypass the fast path.

The fast path is disabled on big-endian platforms.

Callers do not opt in, and the fast path is transparent in every way
users should rely on. One observable change: function wrappers returned
by `library.getFunction`, `library.getFunctions`, and `library.functions`
now have `.length` equal to the declared parameter count rather than
`0`. Code that relied on the previous value will need to be updated.

Author: bengl

lib/ffi.js

@@ -53,6 +53,8 @@ const {
   toArrayBuffer,
 } = internalBinding('ffi');
 
+require('internal/ffi-shared-buffer');
+
 function checkFFIPermission() {
   if (!permission.isEnabled() || permission.has('ffi')) {
     return;