Allow multiple segments in Stringliterals

[pawelsadlo]

This PR adds support for multi-segment literal strings.
for example

val path = root / "foo/bar" 
val qux = "qux"
val path = root / "foo/bar" / "baz" / qux
val path = root / "foo/bar" / "baz/qux"

it also parses .. segments from literal as os.up enabling syntax like:

val path = root / "foo" / ".." / "bar" // equivalent to `root / "foo" / os.up / "bar"`
val path = root / "foo" /  "../bar" // equivalent to `root / "foo" / os.up / "bar"`

non-canonical paths used in literals result in compile-time errors, suggesting usage of canonical paths or removing path segment, eg.

val path = root / "foo/./bar" //suggests "foo/bar"
val path = root / "foo//bar" //suggests "foo/bar"
val path = root / "//foo//bar/./baz" //suggests "foo/bar/baz"

val path = root / ""  //suggests removing the segment
val path = root / "." //suggests removing the segment
val path = root / "/" //suggests removing the segment
val path = root / "//" //suggests removing the segment
val path = root / "/./" //suggests removing the segment

Note:
Its not usable in os-Lib itself, due to the fact that it would lead to macro expansion in the same compilation unit as its definition.

@lihaoyi
there is a little bit of hacking involved:

1. There is a default implicit conversion not being a macro to be used within os library, without this we would have to add a submodule and split the whole project, I'm not even sure if its doable.
2. Needed to turn off acyclic in Path and particular Macro files (also needed to mock acyclic.skipped in case of scala 3).
3. Needed to provide another implicit conversion in ViewBoundImplicit trait because macros turn out to be not avaliable as implicit views, this is needed for ArrayPathChunk and SeqPathChunk to work.

[pawelsadlo]

@lihaoyi could you rerun checks?

[pawelsadlo]

@lihaoyi
could you take a look at checks after [924a02e]?
did you encounter something like this before?

adding more tests[5a49ac2] succeed in (ubuntu-latest, 17) and almost nothing changed since then (i have only added binary compatibility shims)

After last failed workflow I have changed macro to make StringPathChunk and ArrayPathChunk instead of SubPathChunk, but I dont know why SubPathChunk would fail, expecially only on particular environment and not others

[lihaoyi]

@pawelsadlo will take a look!

[pawelsadlo]

@lihaoyi it succeed now, so I dont know if we need to examine it, anyway could do a review?

[lihaoyi]

Will take a look tomorrow

[lihaoyi]

I think we can do a more user-focused error message here, something like

literal path sequence <entire-literal> used in OS-Lib must be in a canonical form, please use <literal-without-leading-slashes> instead

[pawelsadlo]

@lihaoyi
I have provided a NonCanonicalLiteral exception for this case
actually there is also an edge case when sanitizing literal results in empty path, for example //, /., . literals , in such cases i throw:

InvalidSegment(
      literal,
      s"Literal path sequence [$literal] doesn't affect path being formed, please remove it"
    )

If you have any better suggestions for error messages I can change them.

[lihaoyi]

@pawelsadlo looks great, left some comments. If you update the code and update the PR description to have the most up to date state of the PR I think we can merge it

[pawelsadlo]

@lihaoyi could you please rerun checks?

[pawelsadlo]

@lihaoyi let me know if there are any suggestions left

[lihaoyi]

@pawelsadlo looks great, thanks! I merged it, I assume your bank details haven't changed so I'll pay out the bounty to the same account as last time