Remove leading indentation from (some) pod code blocks.

[cfa]

There are numerous instances of =for and =begin code blocks marking already indented code peppered through the documentation. This indentation is preserved in the rendered output, which is typically unintentional. So, for example:

=for code
    say 'hello';

is rendered as

    say 'hello';

rather than

say 'hello';

Although this is a fairly boring commit, it touches a lot of files—hence the PR rather than direct push. Note that If you ignore whitespace (git diff master -w), no content has changed. There're also no test or xtest regresssions.

[AlexDaniel]

Thanks!