add explicit items()/keys()/values() methods

[matthiasdiener]

This appears to speed up these operations substantially.

With this small benchmark:

import immutabledict
import timeit

d_init = {i:i for i in range(10000000)}

d = dict(d_init)
print("dict", timeit.timeit(
    "for k, v in d.items(): s += 1", globals=globals(), number=3, setup="s=0"))

d = immutabledict.immutabledict(d_init)
print("immutabledict", timeit.timeit(
    "for k, v in d.items(): s += 1", globals=globals(), number=3, setup="s=0"))

Before:

$ python dictspeed.py
dict 0.5598837919533253
immutabledict 2.162265417049639

After:

$ python dictspeed.py
dict 0.5607237920630723
immutabledict 0.5689269590564072

The added test fails without this PR.

[corenting]

Sorry for the late response and thanks for the MR!

I cherry-picked the commmit in #267 as there were some formatting changes needed. The MR was merged so I'm closing this one.