Static & dynamic field accessors for product types

[jserranohidalgo]

We can't create column expressions that access fields of product type columns. For instance, given the following product type:

case class User(name: String, age: Int)

we would like to access fields of users as follows:

col[User]("user").getChildSafe("name"): DoricColumn[String]
col[User]("user").getChildSafe("age"): DoricColumn[Int]

Moreover, we would like to get a compilation error if we try to access a non-existing field:

col[User]("user").getChildSafe("surname") // should not compile

Dynamic invocations should be allowed too:

col[User]("user").child.name[String]: DoricColumn[String]
col[User]("user").child.age[Int]: DoricColumn[Int]
col[User]("user").child.surname[String] // should raise a non-existent column error at runtime

[jserranohidalgo]

The approach followed in the Compossible library for implementing records could be of interest for us:

https://github.com/cvogt/compossible/blob/master/src/main/scala/Record.scala#L32

Basically, they use macros for implementing the Dynamic interface.

This may help in simplifying syntax for safe access, i.e. dc.name instead of dc.getChildSafe('name) , and reporting errors (#265)

[jserranohidalgo]

Preliminary work on macro-based implementations of the Dynamic interface here:
https://github.com/jserranohidalgo/doric/tree/jm/struct-macros

Besides the problems of IDEs with whitebox macros, the current proposal in that branch uses the applyDynamic method, which is ugly. More work is needed to use the selectDynamic one.

[jserranohidalgo]

Work is continued on #332