Add specialized Pointer type.

[izulin]

Is your feature request related to a problem? Please describe. Right now pathway tracks just the type Pointer, without the type of the pointer itself.

Describe the solution you'd like It should at least track the type used to construct this pointer, eg Pointer[int,str]. This way it will have (at least) poor-man's type verification of key columns, and it would catch errors such as:

• concating/update_cells between tables with incompatible keys;
• ix_ref'ing into tables using wrong columns as keys.

[izulin]

Should be in a public release soon.