toPath: Make path steps type more formal

[nlepage]

Description

toPath() returns results in the following form :
['foo', 1, [1, 3]]

Determining the type of a path step relies on type assertion.

I think it would be better to have a more formal type for path steps :
[[prop, 'foo'], [index, 1], [slice, 1, 3]]

prop, index and slice being symbols exported by path namespace (see #147).