SP int: sp_modinv fixes for sizes

sp_invmod with even modulus requires a multiplication by modulus. Don't
let modulus overflow result variable 'r'.
Fix allocation of temporary sp_ints to be correct size.
Add test for maximum modulus size in test.c.

Remove leading spaces on functions so git correctly determines which
function has changed.
Put in Thumb code for more sizes of _sp_mul_*().