Speed up Image.getextrema()

[akx]

Speeds up getextrema() -- with more than just hoist and restrict this time, by adding a separate function for the common case of 2-4 band 8bpc images. The single-channel case speeds up pleasantly too by autovectorisation, though.

------------------- benchmark 'extrema': 4 tests, 2 sources --------------------
Name (time in us)                   0001_3a36c3a OPS  0003_796ae94 OPS      ΔOPS
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test_getextrema[1024x1024-RGB]            1,790.4542       13,099.6057   +631.6%
test_getextrema[1024x1024-L]              1,410.9957       17,641.2525  +1150.3%
test_getextrema[1024x1024-RGBA]             687.5769       13,102.1316  +1805.6%
test_getextrema[1024x1024-LA]               494.9731       11,851.6516  +2294.4%
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