Fix issue 12747 std.regex bug in the parser allows reference to open groups.

Check the stack of opened constructs for groups when emit back-refs.
add annotations to biguint
std.stdio: annotate C functions
std.conv: parse not handling some cases
Convert std.datetime.Datetime to use ddoc-ed unit tests.
Reformat some ddoc-ed examples so that they're not so long.
Merge pull request #2170 from DmitryOlshansky/issue-12747

Fix issue 12747 std.regex bug in the parser allows reference to open gro...
Merge pull request #2184 from WalterBright/annotate-stdio

std.stdio: annotate C functions
Merge pull request #2182 from WalterBright/annotate-biguint

add annotations to biguint
Merge pull request #2189 from jmdavis/datetime_ddoc_tests

Convert std.datetime.Datetime to use ddoc-ed unit tests.
std.json: ensure objects contain keys
Merge pull request #2185 from WalterBright/fix-parse

std.conv: parse not handling some cases
Small code reduction in std.ascii.

We can have the comment only once, and it will still show up in the
docs.
Minor stylistic cleanup in std.ascii.
Merge pull request #2196 from jmdavis/ascii

Some minor cleanup in std.ascii
Merge pull request #2186 from Yoplitein/master

std.json: throw exception if a key does not exist
Avoid double lookup in opIndex for std.json.
Merge pull request #2197 from AndrejMitrovic/Fixup2186

Avoid double lookup in opIndex for std.json.