Perf: ⚡ merge if you want to go faster but don't need float support

[lbillingham]

This moves away from the shared itertools implimentations for finding pairs, triplets of the input numbers that sum to a given target.

Instead, we

• 1st expose the underlying $~O^{dimensions}$ nested loops
• trade some extra memory and some $O^{1}$ lookups to give us $~O^{dimensions-1}$
• get a >= 170x speedup in our benchmarks

However, we:

• loose the ability to properly work with floating point input
  • the fast lookup uses hasing and hashing floats gets weird due to floating point equality
• can't trivially extend to higher-dimension problems: 4-element-tuples etc.

I've moved the float input tests out the their own file and away from the CI test path

Performance benchmarks

with these changes:

$ make benchmark
tests/performance_check.py ..                                                                                                                             [100%]

-------------------------------------------------------------------------------------------- benchmark: 2 tests --------------------------------------------------------------------------------------------
Name (time in us)               Min                   Max                  Mean              StdDev                Median                 IQR            Outliers          OPS            Rounds  Iterations
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
test_input1_pairs           22.1660 (1.0)        166.3790 (1.0)         23.6089 (1.0)        5.0170 (1.0)         23.0000 (1.0)        0.5123 (1.0)        87;521  42,356.8183 (1.0)        7677           1
test_input1_triplets     1,994.8000 (89.99)    3,561.1120 (21.40)    2,152.1272 (91.16)    204.1428 (40.69)    2,033.1040 (88.40)    299.1878 (584.07)       41;4     464.6565 (0.01)        341           1
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

itertools, but non-float supporting version

$ make benchmark
tests/performance_check.py ..                                                                                                      [100%]

------------------------------------------------------------------------------------- benchmark: 2 tests ------------------------------------------------------------------------------------
Name (time in ms)             Min                 Max                Mean            StdDev              Median               IQR            Outliers       OPS            Rounds  Iterations
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
test_input1_pairs          2.8727 (1.0)        4.2386 (1.0)        3.1265 (1.0)      0.1638 (1.0)        3.1067 (1.0)      0.1888 (1.0)          78;9  319.8414 (1.0)         326           1
test_input1_triplets     211.6325 (73.67)    213.3950 (50.35)    212.4042 (67.94)    0.6555 (4.00)     212.2717 (68.33)    0.8081 (4.28)          2;0    4.7080 (0.01)          5           1
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
=========================================================== 2 passed in 3.59s ============================================================

Note the change in units this branch is in microseconds, the itertools version is in milliseconds