feat: Constify many functions

[kleinesfilmroellchen]

This is primarily aimed at making SourceSpan and SourceOffset usable in const contexts.

Constifiable functions were found with the clippy::missing_const_for_fn lint, though it reported at least two false positives.

Fixes #261

[kleinesfilmroellchen]

Note: I couldn't test this with MSRV or any versions before 1.69 due to this error:

❯ cargo +1.58.0 nextest run      
    Updating crates.io index
error: failed to select a version for the requirement `aho-corasick = "^1.0.0"`
candidate versions found which didn't match: 0.7.20, 0.7.19, 0.7.18, ...
location searched: crates.io index
required by package `regex v1.8.1`
    ... which satisfies dependency `regex = "^1.5"` (locked to 1.8.1) of package `miette v5.8.0 (miette)`
error: command `'\\?\~\.rustup\toolchains\1.58.0-x86_64-pc-windows-msvc\bin\cargo.exe' test --no-run --message-format json-render-diagnostics` exited with code 101

[zkat]

looks like this is relying on stuff that was unstable as of the current MSRV (1.56.0). Considering you can't seem to run this on earlier versions to test, maybe it's better to mark this as "breaking" and include it with miette 6.0 instead?

[kleinesfilmroellchen]

looks like this is relying on stuff that was unstable as of the current MSRV (1.56.0). Considering you can't seem to run this on earlier versions to test, maybe it's better to mark this as "breaking" and include it with miette 6.0 instead?

I didn't change my toolchain so these failures are strange. I will just throw away all the constification that cannot be done right now.