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.
Added std.experimental
Merge pull request #2190 from Orvid/stdexperimental

Added std.experimental
Moved note.md into the right directory
Don't assume va_list is void*
Merge pull request #2191 from Orvid/stdexperimental

Moved note.md into the right directory
Fixup spelling mistake.
Merge pull request #2193 from AndrejMitrovic/master

Fixup spelling mistake.
Merge pull request #2192 from yebblies/valisttype

Don't assume va_list is void*
Added an in operator to std.json.JSONValue. Tested when const/non-const
Merge pull request #2194 from markisaa/jsonValueInOp

Issue 12784: Added an in operator to std.json.JSONValue. Tested when const/non-const
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.