Inconsistent use of `_` to separate words in method names
Sometimes, method names contain underscores to separate words (`to_string`) and sometimes not (`tolist`). This catches me of regularly. Particulary annoying are the cases where the same prefix (eg. `to`) sometimes takes an underscore and sometimes not.

Is this going to change for future major versions?

Example: `dir(Index)`
- Without underscore between words, but the compound is well-established: 'argmax', 'argmin', 'argsort', 'nbytes', 'ndim', 'nlevels', 'nunique', 'putmask'
- Without underscore: 'astype', 'fillna', 'hasnans', 'isin', 'itemsize', 'searchsorted', 'sortlevel', 'tolist'
- With underscore: 'drop_duplicates', 'get_duplicates', 'get_indexer', 'get_indexer_for', 'get_indexer_non_unique', 'get_level_values', 'get_loc', 'get_slice_bound', 'get_value', 'get_values', 'groupby', 'has_duplicates', 'holds_integer', 'inferred_type', 'is_all_dates', 'is_boolean', 'is_categorical', 'is_floating', 'is_integer', 'is_lexsorted_for_tuple', 'is_mixed', 'is_monotonic', 'is_monotonic_decreasing', 'is_monotonic_increasing', 'is_numeric', 'is_object', 'is_type_compatible', 'is_unique', 'memory_usage', 'set_names', 'set_value', 'slice_indexer', 'slice_locs', 'sort_values', 'sym_diff', 'symmetric_difference', 'to_datetime', 'to_native_types', 'to_series', 'value_counts'
