rustdoc: Fix testing indented code blocks

The collapse/unindent passes were run in the wrong order, generating different
markdown for indented tests.
rustdoc: Don't inject `extern crate std`.

No need to duplicate the compiler's work!

Closes #14999
std: Remove dual export of `Show`

Closes #14996
auto merge of #15000 : alexcrichton/rust/fix-rustdoc-tests, r=huonw

Commits have the descriptions.