diesel: ad-hoc queryable structure generation

[CertainLach]

Currently, immigrant-diesel can only generate structure to select all rows from the table.

Adding ability to specify multiple sets of query fields on schema level is infeasible and will not make this easier to use.

But what about generating a macros, which then can generate queryable struct for the given subset of fields?

from_row!(AdHocData = MyTable(field1, field2))
=>
#[diesel(table = "db_my_table")]
struct AdHocData {
   field1: ...
   field2: ...
}