Implement Postgres.Call

[jwp]

Currently, direct function calls cannot leverage SRFs or functions that keep state.

An explicit Call object to manage that state is necessary. This is, essentially, objectified fcinfo.

[jwp]

Postgres.Call needs to have some intelligence in order to distinguish between SRFs and regular functions; this also has some impact on the supported interfaces:

SRFs will only support next
Non-SRFs will support call()

VPC-SRFs will likely be the supported mode, and each call will need it's own MemoryContext.

Also, consider an optimization for SRFs that return a Call() like Postgres.Cursor() returns.