Cache generated grammar regexps to improve performance

[zmughal]

This improves the performance of the test suite with a 30%-70% reduction
in execution time depending on how heavily the code generates grammar
regexps (with the greatest reduction for the postfixderef and
substituteandreturn tests) which can be seen in the following runs of
prove --timer:

Before:

t/plugin-coresignatures.t ....... ok     3640 ms ( 0.00 usr  0.00 sys +  3.53 cusr  0.10 csys =  3.63 CPU)
t/plugin-definedor.t ............ ok     1586 ms ( 0.00 usr  0.00 sys +  1.52 cusr  0.07 csys =  1.59 CPU)
t/plugin-ellipsis.t ............. ok      192 ms ( 0.00 usr  0.00 sys +  0.17 cusr  0.02 csys =  0.19 CPU)
t/plugin-packageblock.t ......... ok      281 ms ( 0.00 usr  0.00 sys +  0.24 cusr  0.04 csys =  0.28 CPU)
t/plugin-packageversion.t ....... ok      979 ms ( 0.01 usr  0.00 sys +  0.92 cusr  0.06 csys =  0.99 CPU)
t/plugin-postfixderef.t ......... ok    10813 ms ( 0.00 usr  0.00 sys + 10.59 cusr  0.23 csys = 10.82 CPU)
t/plugin-sigify.t ............... ok      394 ms ( 0.00 usr  0.00 sys +  0.37 cusr  0.02 csys =  0.39 CPU)
t/plugin-skt.t .................. ok      534 ms ( 0.00 usr  0.00 sys +  0.51 cusr  0.03 csys =  0.54 CPU)
t/plugin-state.t ................ ok     1713 ms ( 0.00 usr  0.00 sys +  1.63 cusr  0.08 csys =  1.71 CPU)
t/plugin-substituteandreturn.t .. ok    70323 ms ( 0.00 usr  0.00 sys + 70.09 cusr  0.24 csys = 70.33 CPU)
t/simple.t ...................... ok      356 ms ( 0.00 usr  0.00 sys +  0.33 cusr  0.02 csys =  0.35 CPU)
t/submatch-rule.t ............... ok      681 ms ( 0.00 usr  0.00 sys +  0.66 cusr  0.03 csys =  0.69 CPU)

All tests successful.
Files=12, Tests=79, 92 wallclock secs ( 0.03 usr  0.02 sys + 90.56 cusr  0.94 csys = 91.55 CPU)
Result: PASS

After:

t/plugin-coresignatures.t ....... ok     1825 ms ( 0.01 usr  0.00 sys +  1.78 cusr  0.04 csys =  1.83 CPU)
t/plugin-definedor.t ............ ok      890 ms ( 0.00 usr  0.00 sys +  0.83 cusr  0.06 csys =  0.89 CPU)
t/plugin-ellipsis.t ............. ok      150 ms ( 0.00 usr  0.00 sys +  0.13 cusr  0.02 csys =  0.15 CPU)
t/plugin-packageblock.t ......... ok      221 ms ( 0.00 usr  0.00 sys +  0.20 cusr  0.02 csys =  0.22 CPU)
t/plugin-packageversion.t ....... ok      401 ms ( 0.00 usr  0.00 sys +  0.36 cusr  0.04 csys =  0.40 CPU)
t/plugin-postfixderef.t ......... ok     5119 ms ( 0.00 usr  0.00 sys +  4.98 cusr  0.15 csys =  5.13 CPU)
t/plugin-sigify.t ............... ok      385 ms ( 0.00 usr  0.00 sys +  0.34 cusr  0.04 csys =  0.38 CPU)
t/plugin-skt.t .................. ok      428 ms ( 0.00 usr  0.00 sys +  0.38 cusr  0.05 csys =  0.43 CPU)
t/plugin-state.t ................ ok      795 ms ( 0.00 usr  0.00 sys +  0.75 cusr  0.05 csys =  0.80 CPU)
t/plugin-substituteandreturn.t .. ok    22202 ms ( 0.00 usr  0.00 sys + 22.15 cusr  0.05 csys = 22.20 CPU)
t/simple.t ...................... ok      329 ms ( 0.00 usr  0.00 sys +  0.29 cusr  0.04 csys =  0.33 CPU)
t/submatch-rule.t ............... ok      440 ms ( 0.00 usr  0.00 sys +  0.42 cusr  0.03 csys =  0.45 CPU)

All tests successful.
Files=12, Tests=79, 33 wallclock secs ( 0.04 usr  0.01 sys + 32.61 cusr  0.59 csys = 33.25 CPU)
Result: PASS