Memoize getAllPermutationsOfLeetSubstitutions

[ghost]

Textbook example of preventing exponential running time in recursive function.

Fixes: #15.

[nbutton23]

This breaks the test in matching_test.go

[nbutton23]

Fails test.
Error Trace: matching_test.go:166
Error: "[pa$sw0rd passw0rd password pa$sword p4ssw0rd p4$sword]" should have 7 item(s), but has 6
Messages: There should be 6 permutations for p4$sw0rd
Error Trace: matching_test.go:173
Error: "[pa$$w0rd pas$w0rd passw0rd password pas$word pa$sw0rd pa$$word p4s$w0rd p4$sw0rd p4$$word]" should have 15 item(s), but has 10
Messages: Check my math 2222 - 1 p4$$w0rd
Error Trace: matching_test.go:179
Error: "[i337 ie37 iee7 ieel ieet ie3l ie3t i3e7 i33l i33t l337 1e37 13e7 133l 133t]" should have 35 item(s), but has 15
Messages: check my math 3223 -1