Fix compilation for 32-bit targets

[wg]

Hello, thank you for rusty_v8! Building for the 32-bit armv7-unknown-linux-musleabihf
target gives the following compilation error due to some assumptions about the
layout of the C++ v8::ScriptCompiler::Source class:

  ../../../../src/binding.cc:43:50: error: static assertion failed: Source size mismatch
     43 | static_assert(sizeof(v8::ScriptCompiler::Source) <= sizeof(size_t) * 8,

This assert matches the corresponding Rust struct definition pub struct Source([usize; 8]);
on 64-bit platforms where the size of the fields plus padding mean
sizeof(v8::ScriptCompiler::Source) == 64.

However on 32-bit ARM sizeof(v8::ScriptCompiler::Source) == 36 but
size_of::<[usize; 8]>() == 32.

This PR updates the layout of the Rust v8::script_compiler::Source struct
to match that of the C++ class and updates the static_assert to check that the size
of the C++ class matches the expected aligned sum of field sizes.

With this change I'm able to build & run a program that links to rusty_v8 on
armv7-unknown-linux-musleabihf, as well as aarch64-unknown-linux-musl, and
x86_64-unknown-linux-musl.

[CLAassistant]

All committers have signed the CLA.

[RaisinTen]

The indentations don't look quite right to. You could fix these by running cargo fmt.

[wg]

Thanks! I've fixed this. And thank you for kicking off the workflow run @littledivy.

[littledivy]

LGTM

cc @bartlomieju @piscisaureus