Create a direct lookup hash for ->isa() lookup, by retaining the

Create a direct lookup hash for ->isa() lookup, by retaining the
de-duping hash used by S_mro_get_linear_isa_dfs(). Provide a new
function Perl_get_isa_hash() to lazily retrieve this. (Which could
actually be static if S_isa_lookup() and Perl_sv_derived_from()
moved into mro.c.) Make S_isa_lookup() use this lookup hash in place
of a linear walk of the linear isa. This should turn isa lookups from
O(n) to O(1), which should make heavy users of ->isa() faster.
(eg PPI, and hence Perl Critic).

p4raw-id: //depot/perl@34354