`Imm` representation ideas

[Firestar99]

From Rust-GPU/rust-gpu#379 (comment) since @eddyb asked me to open a separate issue about it:

From what I could gleam from spirt source:
[...]

• Imm::Short(u32) is sufficient for all usual operands, as they are all just u32-sized enums
• Exception: string literals, for which you use one Imm::LongStart with many Imm::LongCont[inued] to encode a stream of N-many bytes
  • I have not found any other case using Long

Thoughts

• With Imm::Long* rarely used but Imm::Short ubiquitous, I wonder if an Imm::unwrap_short() could make code more readable? And maintainable, since it would allow you to replace the backing datastructure later without much code breakage.
• With Imm::Long being so rare and I assume untouched by spirt, I wonder if a plain Vec<u8> or &'cx [u8] is more efficient and simpler? That would also make OpString only have 2 Imms, which makes it fit in Inst.imms: SmallVec<[Imm; 2]> and only be one indirection to access, as previously.

pub enum Imm {
    Short(spec::OperandKind, u32),
    Long(spec::OperandKind, Vec<u32>),
}

Another idea from Imm docs:

spirt/src/spv/mod.rs

Lines 97 to 98 in 43524d7

	// FIXME(eddyb) consider replacing with a `struct` e.g.:
	// `{ first: bool, last: bool, kind: OperandKind, word: u32 }`