feat(ops): Basic buffer support for op2

[mmastrac]

Adds:

• #[buffer] &[u8]
• #[buffer] &mut [u8]

TODO:

• #[buffer] *const u8 (all primitive types) -- not recommended but used in FFI
• #[buffer] *mut u8 (all primitive types)
• #[buffer] bytes::Bytes
• #[buffer] JSBuffer
• #[buffer] V8Slice
• #[buffer] Vec<..>
• #[buffer] Box<[..]>

TODO: Alternative buffer modes:

• #[buffer(shared)] (the default)
• #[buffer(copy)]
• #[buffer(detach)]

[aapoalas]

thought: It's possible for V8 ArrayBuffers to be created from external pointers that do not follow the normal alignment rule of V8 ABs (code says always aligned to 4 bytes, experience seems to suggest this is 8 bytes for 64 bit environments). In these cases the TypedArrays will then happily allow for themselves to be created at odd offsets. This needs to be remembered when eg. a Float64Array is converted into a Rust slice.

[mmastrac]

thought: It's possible for V8 ArrayBuffers to be created from external pointers that do not follow the normal alignment rule of V8 ABs (code says always aligned to 4 bytes, experience seems to suggest this is 8 bytes for 64 bit environments). In these cases the TypedArrays will then happily allow for themselves to be created at odd offsets. This needs to be remembered when eg. a Float64Array is converted into a Rust slice.

Definitely -- this is automatic in the fast path, but we should throw alignment errors if this happens in the slow path as well. Right now the existing f32/f64/u32/etc ops probably "work" on x64 and will segfault on ARM64.

[bartlomieju]

LGTM