Make FileId non-zero

[etaoins]

When porting from codespan 0.3 Arret I needed to track the FileId separately from the Span now that the file isn't implied in the span itself. Having a simple struct like this works for 90% of the cases:

struct Span {
  file_id: codespan::FileId,
  codespan_span: codespan::Span,
}

However, this requires that every time we parse any source we require a FileId which requires a codespan::Files to create. This is overkill in a couple of cases:

1. Unit tests that just need to parse simple Arret strings as a shorthand for building our AST

2. Syntax highlighting in the REPL

The compromise in etaoins/arret#160 is to use Option<FileId>. I'm not 100% happy with that as the ultimate solution but it was the easiest path to take during porting. However, this makes the size of the above struct 13 bytes which will typically be rounded up to 16 bytes for alignment and padding.

We can use std::num::NonZeroU32 instead and offset all of our indexes by 1. This allows the Rust compiler to use 0 to represent the None case and bring the size down to 12 bytes. Because the struct only needs an alignment of 4 bytes this is more likely to stay as 12 bytes in most contexts.

A nicer solution would be if Rust had e.g. NonMaxU32 but it appears the Rust team instead wants to use const generics so arbitrary niche values can be specified.

[brendanzab]

Awesome, I've been thinking about this too. This might make #138 easier too. 🤔

[etaoins]

@brendanzab

I think so. From what I understand the compiler heuristics are pretty clever. Having a FileId anywhere in a struct should make an Option of that struct "free" from a memory layout perspective.