Toolstate publication only runs if the channel is "nightly" and
previously the toolstate builders did not know that the channel was
nightly (since they are not dist builders).

A look through bootstrap seems to indicate that nothing should directly
depend on the channel being set to `-dev` on the test builders, though
this may cause some problems with UI tests (if for some reason they're
dumping the channel into stderr), but we cannot find evidence of such so
hopefully this is fine.

We were inserting *before* the existing newline, so we should prepend it
not append it to our inserted string.

Add more delegations to the fmt docs and add doctests

HI,
this is a continuation to rust-lang#67021
I replaced the `Debug` example with one that use the `Debug*` helpers so that padding etc will work too.
I also added asserts for the doctests as @RalfJung asked :)

The only thing I left with the `write!` macro is the `Display` example as I didn't know if there's a better way to do that.

r? @QuietMisdreavus

…xcrichton

add string.insert benchmarks

This adds benchmarks for `String::insert` and `String::insert_str`

…ietroalbini

Set release channel on non-dist builders

Toolstate publication only runs if the channel is "nightly" and
previously the toolstate builders did not know that the channel was
nightly (since they are not dist builders).

A look through bootstrap seems to indicate that nothing should directly
depend on the channel being set to `-dev` on the test builders, though
this may cause some problems with UI tests (if for some reason they're
dumping the channel into stderr), but we cannot find evidence of such so
hopefully this is fine.

r? @pietroalbini

Fix internal documentation typo

r? @Centril

…ntril

Fix toolstate history format

We were inserting *before* the existing newline, so we should prepend it
not append it to our inserted string.

…r=alexcrichton

Correct the todo! stabilization version

None