ENH: improve partial key indexing performance
Indexing for entries in a series or dataframe with a multi-index has dramatically worse performance when using partial keys.

For example, for a series with a 3-level multi-index, `series[x]` is dramatically slower than `series[(x,y,z)]`. Please see the session below for a minimal reproduction:

```In [1]: %paste
from datetime import datetime
import pandas as pd
import numpy as np
import timeit


dates = pd.date_range('1997-01-01', '2020-12-31')
cats = list('abcdefghijklmnop')

multi = pd.MultiIndex.from_product([dates, cats, cats])

series = pd.Series(np.random.rand(len(multi)), index=multi)
date = datetime(year=2020, month=1, day=1)

# generate cache
series.loc[(date, 'a', 'a')]

repeats = 1000
len(series)
## -- End pasted text --
Out[1]: 2244096

In [2]: # Performance of indexing with full keys

In [3]: timeit.timeit(lambda: series.loc[(date, 'a', 'a')], number=repeats)
Out[3]: 0.08493588299825205

In [4]: # Performance of indexing with partial keys

In [5]: timeit.timeit(lambda: series.loc[date], number=repeats)
Out[5]: 4.771169905001443
```

From looking at the code, it seems that `BaseMultiIndexCodesEngine` (pandas/_libs/index.pyx) essentially creates a hash table (a `libindex.UInt64Engine`) with entries for each full key. This makes full-key lookups fast. Meanwhile, for partial key lookups on a sorted index, `MultiIndex._get_level_indexer` will do two binary searches for each provided level of the key.

Instead of a flattened-index approach (again, via `libindex.UInt64Engine`), it would be *excellent* if Pandas implemented nested indeces (nested hash tables, if you will) for this. This would have a small impact on the performance of full key lookups (current lookup complexity is O(1), new complexity would be O(number_of_levels)). However, partial key lookups would greatly benefit, going from O(log(size_of_series)) to O(number_of_levels_in_key).