Allow conversions from string to complex types

Assuming the format of representing complex types in string, we should be able to convert strings to complex types.

This can happen in several cases:

1. Serializing a string into a column of a table which is complex-typed.
2. Sending a string into a UDF/GenericUDF which accepts complex types.
3. User explicitly do type casting using syntax like CAST(mystring AS array<int>)

We should assume a default format (maybe JSON, or maybe ^A, ^B, ^C format).

This makes it easier for people to produce complex types from their transform scripts.