Fix #9819: Allow access to named tuple's names

Adds a `fieldNames` alias to `std.typecons.Tuple` to return the user-supplied
names.  The actual fields are in an expression tuple, so the
user-supplied names are simply aliases, which previous to this PR, were
not accessible.

https://issues.dlang.org/show_bug.cgi?id=9819

Fix Issue 9819
Rename local variables to avoid conflict with local imports
Slight improvement to enforceEx's constraints and its documentation.
Merge pull request #2232 from economicmodeling/fix_9819

Fix 9819: Allow access to named tuple's names.
Merge pull request #2265 from jmdavis/exception

Slight improvement to enforceEx's constraints and its documentation.
Remove deprecated module std.metastrings

This module has been deprecated and scheduled for removal in March 2014.
It is now June 2014.
Merge pull request #2267 from quickfur/del_metastrings

Remove deprecated module std.metastrings
Merge pull request #2262 from 9rnsr/fix_imports

Rename local variables to avoid conflict with local imports