Allows use with `Option` and custom `Result`-like types.

The dummy impl should ensure the same type checking behaviour as having other (real) Carrier impls.

Solaris linker doesn't support the `--retain-symbols-file` option,
so this patch provides version scripts as an alternative on that
platform.

Based on following what happens in CString::new("string literal"):

1. Using `Into<Vec<u8>>`, a Vec is allocated with capacity exactly equal
   to the string's input length.
2. By `v.push(0)`, the Vec is grown to twice capacity, since it was full.
3. By `v.into_boxed_slice()`, the Vec capacity is shrunk to fit the length again.

If we use `.reserve_exact(1)` just before the push, then we avoid the
capacity doubling that we're going to have to shrink anyway.

Growing by just 1 byte means that the step (2) is less likely to have to
move the memory to a larger allocation chunk, and that the step (3) does
not have to reallocate.

Carrier trait (third attempt)

This adds a `Carrier` trait to operate with `?`. The only public implementation is for `Result`, so effectively the trait does not exist, however, it ensures future compatibility for the `?` operator. This is not intended to be used, nor is it intended to be a long-term solution.

Although this exact PR has not been through Crater, I do not expect it to be a breaking change based on putting numerous similar PRs though Crater in the past.

cc:
* [? tracking issue](#31436)
* [previous PR](#35056)
* [RFC issue](rust-lang/rfcs#1718) for discussion of long-term Carrier trait solutions.

r? @nikomatsakis

Fix linker on Solaris/Illumos

This patch provides a fix for the `GnuLinker::export_symbols` function that currently relies on a `--retain-symbols-file` option which is not supported by the Solaris & Illumos linker.

Instead, a [version script](https://www.gnu.org/software/gnulib/manual/html_node/LD-Version-Scripts.html) is used on this platform to achieve the same goal. Here's an example of a similar approach in LLVM's CMake script: https://github.com/llvm-mirror/llvm/blob/master/cmake/modules/AddLLVM.cmake#L88-L94.

Perhaps other platforms like OpenBSD could benefit from this as well. /cc @semarie

Check that executable file is in-tree before failing tidy check

I silenced stdout and stderr for ls-files, not sure if that's appropriate (is `make tidy` intended to give debugging information)? Otherwise it prints each file it find to stdout/stderr, which currently prints nothing (only executable files are checked).

I have not done major testing regarding the behavior of ls-files when the file is ignored, but judging by the man page everything should be fine.

I've duplicated the code which makes the path git-friendly from the `Cargo.lock` checking code; I can extract that into a common helper if wanted (it's only two lines).

Fixes #35689.

cstring: avoid excessive growth just to 0-terminate

Based on following what happens in CString::new("string literal"):

1. Using `Into<Vec<u8>>`, a Vec is allocated with capacity exactly equal
   to the string's input length.
2. By `v.push(0)`, the Vec is grown to twice capacity, since it was full.
3. By `v.into_boxed_slice()`, the Vec capacity is shrunk to fit the length again.

If we use `.reserve_exact(1)` just before the push, then we avoid the
capacity doubling that we're going to have to shrink anyway.

Growing by just 1 byte means that the step (2) is less likely to have to
move the memory to a larger allocation chunk, and that the step (3) does
not have to reallocate.

Addresses part of #35838