[FEATURE REQUEST] Feature flag for `i64` support

[ggirelli]

Currently metis-sys forces 32-bit depth for IDX_SIZE and REAL_SIZE

metis-rs/metis-sys/build.rs

Lines 4 to 8 in 410f512

	#[cfg(feature = "vendored")]
	const IDX_SIZE: usize = 32;
	#[cfg(feature = "vendored")]
	const REAL_SIZE: usize = 32;

It would be very nice to support 64-bit depth via a feature flag.

[gfaster]

That's probably not easily doable, since idx and real width are configuration constants, 32-bit and 64-bit features would be mutually exclusive, or incur a major performance hit from converting slices.

[ggirelli]

Oh, yes. I was thinking of a feature flag, so the library would build with support for either 32-bit or 64-bit, not both.