Merge support for UTF8 symbols

This branch makes symbols support UTF8 internally, which means that
Unicode is supported properly at the perl level.  So ${"\xff"} will
give you the same scalar, regardless of the internal encoding of the
string.  Also, many  parts of the core are now nul-clean, too, as a
result of the UTF8 changes, which means that ‘$m = "a\0b"; foo->$m’
will try to call the method named "a\0b", instead of just "a".
Details follow.

• New API functions:
  Many of these take a _flags parameter, which accept the
  SVf_UTF8 flag.
  • HvNAMELEN
  • HvNAMEUTF8
  • HvENAMELEN
  • HvENAMEUTF8
  • gv_init_pv(n)/sv
  • gv_fetchmeth_pv(n)/sv
  • gv_fetchmeth_pv(n)/sv_autoload
  • gv_fetchmethod_pv(n)/sv_flags — may change
  • gv_autoload_pv(n)/sv
  • newGVgen_flags
  • sv_derived_from_pv(n)/sv
  • sv_does_pv(n)/sv
  • whichsig_pv(n)/sv
• New internal functions:
  • GvNAMEUTF8
  • GvENAMELEN
  • GvENAME_HEK
  • CopSTASH_flags
  • CopSTASH_flags_set
  • PmopSTASH_flags
  • PmopSTASH_flags_set
  • sv_sethek
• Parts of Perl that handle Unicode symbol names correctly:
  • Method names (including those passed to ‘use overload’)
  • Typeglob names (including variable and filehandle names)
  • Package names
  • Constant subroutine names (not nul-clean yet)
  • goto
  • Symbolic dereferencing
  • Second argument to bless() and tie()
  • Return value of ref()
  • Package names returned by caller()
  • Subroutine prototypes
  • Attributes
  • Warnings and error messages that mention filehandles, packages,
    methods, variables, constant values, subroutines, symbolic refer-
    ences, format names and subroutine prototypes
• Parts of Perl that now handle embedded nuls correctly:
  • Method names
  • Typeglob names (including filehandle names)
  • Package names
  • Autoloading
  • Return value of ref()
  • Package names returned by caller()
  • Filehandle warnings
  • Typeglob elements (*foo{"THING\0stuff"})
  • Signal names
  • Warnings and error messages that mention (yes, it’s the same list
    as above) filehandles, packages, methods, variables, constant val-
    ues, subroutines, symbolic references, format names and subroutine
    prototypes
• Other bug fixes
  • *{é} now treats é as the name of the glob (the usual implicit
    quoting), instead of treating it as a bareword (strict-unsafe)
    or function call.  *{é} used to be equivalent to *{+é}, in
    other words.
• Modified modules:
  • constant has been modified not to apply the workaround for the bug
    that this branch fixes, if that workaround does not apply.
  • attributes has been modified as part of making Unicode attri-
    butes work.
  • XS::APItest
  • mro, as part of making method lookup account for Unicode.
• Side effects
  • Blessing into "\0" no longer causes ref() to return false.
  • *{"*é::..."} is now equivalent to *{"é::..."}, just as
    *{"*e::..."} is equivalent to *{"e::..."}.  Previously, the * was
    only stripped if followed by [A-Za-z].
  • $é is now subject to ‘Used only once’ warnings.  It used to be
    exempt, as the code that checked the named considered it a punctu-
    ation variable.