add .map, .flat, .concise, and .summary for Backtrace

[lukasvalle]

add .map, .flat, .concise, and .summary for Backtrace as list missed in #2632

[JJ]

Please check out the comments I've made.

[Altai-man]

Let's see. Please, wait a little, I will build docs now and will write some explanations...

[Altai-man]

Ok, so firstly I built docs using instructions at https://github.com/perl6/doc#building-the-html-documentation
Secondly, I had to install cpan-minus and install Mojolicious dependency to run it locally.

Then make run and opening the page in browser at localhost:3000 address.

^ here you can see how your changes look like (without highlighting enabled): the explanation lines and output are placed in a big code block, which is not good.

To improve the situation, we can:
1)Remove indentation from lines that are not planned to be in the code block, but rather that should be displayed as normal text(such as "This program" and "results in"), as was suggested.
2)The program code indentation ls left as is.
3)The output is not a Perl 6 code, so should be marked as a code(to get monospace font), but text, so we mark it with =for code :lang<text> directive.

For example, one of methods after an edit looks this way:

    multi method map(Backtrace:D: &block --> Seq:D)

It invokes &code for each element and gathers the return values in a sequence and returns it.

This program:

    sub inner { Backtrace.new.map({ say "{$_.file}: {$_.line}" }); }
    sub outer { inner; }
    outer;

results in:

=for code :lang<text>
SETTING::src/core/Backtrace.pm6: 85
SETTING::src/core/Backtrace.pm6: 85
test.p6: 1
test.p6: 2
test.p6: 3
test.p6: 1

=head2 method flat

Note that for the text block, indentation is absent, because =for code directive already treats the next lines as code and if there will be indentation, it will be present in the rendered docs, which is less than awesome.

After another make html-nohighlight and make run, I can check the changes:

^ results. With highlighting on the real site enabled, there will be a clear separation between the code and the result's listing.

I have fixed this pull request committing the changes, so you can check them.

Thanks a lot for your contribution! If there are any questions, don't hesitate to ask.

I have sent a pull request with my changes and this PR can be merged now.