Support basic `CREATE TABLE` statements

[despawnerer]

Tentative syntax:

create_table("Film")
    .columns((
        ("code", "char(5)").primary_key(),
        ("imdb_id", "char(40)").unique(),
        ("title", "varchar(40)").not_null(),
        ("did", "integer").not_null(),
        ("date_prod", "date"),
        ("kind", "varchar(10)"),
        ("len", "interval hour to minute").default("0"),
    ))

• Duplicate or conflicting constraints on columns should not be allowed (i.e. doing .unique().unique() or .null().not_null(). Achievable at compile-time with some generics abuse.
• Creating a statement without columns should not be allowed (also easily achievable at compile time)
• Basic table constraints
  • UNIQUE
  • PRIMARY KEY (wonder if it would be possible to check that there's no primary key column at compile time?)
• Basic indexes support
• Documentation

[despawnerer]

First attempt in 789b744 takes care of the basic structure