Re-order intrpvar.h to minimise holes in the interpreter struct.

Holes were created by commit f59909ab8dad6ceb (April 2012) which removed
PL_reginterp_cnt, commit 7dc8663964c66a69 (Nov 2012) which removed
PL_rehash_seed_set, and commit 8936b48a49448f4e (Dec 2012) which removed
PL_glob_index.

There is still an unavoidable U16 sized hole on the default threaded
configuration on x86_64. (U8 if PERL_SAWAMPERSAND is defined).
Rename PL_interp_size_5_16_0 to PL_interp_size_5_18_0.