perf(ext/ffi): Optimise common pointer related APIs

[aapoalas]

Optimises a few of the more common pointer related APIs:

1. Deno.UnsafePointer.of
2. Deno.UnsafePointerView#getCString

Both optimisations are just by-now standard op(v8) usages.

Deno.UnsafePointer.of

Before:

benchmark                  time (avg)             (min … max)       p75       p99      p995
------------------------------------------------------------- -----------------------------
Deno.UnsafePointer.of  253.92 ns/iter (219.88 ns … 351.89 ns) 260.71 ns 343.19 ns 351.32 ns
Mesmeria% ./target/release/deno bench --unstable -A ./test_ffi/tests/bench.js --filter UnsafePointer
cpu: AMD Ryzen 5 3600 6-Core Processor
runtime: deno 1.23.3 (x86_64-unknown-linux-gnu)

After:

benchmark                  time (avg)             (min … max)       p75       p99      p995
------------------------------------------------------------- -----------------------------
Deno.UnsafePointer.of     205 ns/iter (185.63 ns … 255.27 ns) 210.14 ns  240.7 ns 246.87 ns
Mesmeria% ./target/release/deno bench --unstable -A ./test_ffi/tests/bench.js --filter UnsafePointer
cpu: AMD Ryzen 5 3600 6-Core Processor
runtime: deno 1.23.3 (x86_64-unknown-linux-gnu)

Around 20% performance improvement, from ~250ns to ~200ns.

Deno.UnsafePointerView#getCString

Before:

benchmark                              time (avg)             (min … max)       p75       p99      p995
------------------------------------------------------------------------- -----------------------------
Deno.UnsafePointerView#getCString   264.1 ns/iter (229.11 ns … 350.57 ns) 269.65 ns 347.72 ns 350.26 ns
Mesmeria% ./target/release/deno bench --unstable -A ./test_ffi/tests/bench.js --filter getCString
cpu: AMD Ryzen 5 3600 6-Core Processor
runtime: deno 1.23.3 (x86_64-unknown-linux-gnu)

After:

benchmark                              time (avg)             (min … max)       p75       p99      p995
------------------------------------------------------------------------- -----------------------------
Deno.UnsafePointerView#getCString  211.06 ns/iter (173.44 ns … 306.52 ns) 211.41 ns 292.55 ns 294.84 ns
Mesmeria% ./target/release/deno bench --unstable -A ./test_ffi/tests/bench.js --filter CString
cpu: AMD Ryzen 5 3600 6-Core Processor
runtime: deno 1.23.3 (x86_64-unknown-linux-gnu)

Around 20% performance improvement, from ~265ns to around ~210ns.