Merge ColumnOption::{Null, NotNull} into a single option

[devgony]

To resolve #969,

Goal

Add nullable field To ColumnDef

Remove redundant ColumnOption::{NotNull, Null}

pub struct ColumnDef {
    pub name: String,
    pub data_type: DataType,
+   pub nullable: bool,
    pub options: Vec<ColumnOption>,
}

pub enum ColumnOption {
-   Null,
-   NotNull,
    Default(Expr),
    Unique { is_primary: bool },
}

impl ColumnDef {
-   pub fn is_nullable(&self) -> bool {
-       self.options
-           .iter()
-           .any(|option| option == &ColumnOption::Null)
-   }
..

Todo

• Add nullable field to ColumnDef
• Remove ColumnOption::{NotNull, Null},
• ColumnDef.is_nullable

Next

• Modify default nullable to true
• Should handle primary_key(NotNull) and unique only (Null) properly

[coveralls]

Pull Request Test Coverage Report for Build 3421124370

• 77 of 77 (100.0%) changed or added relevant lines in 11 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 98.497%

---

Files with Coverage Reduction	New Missed Lines	%
core/src/executor/validate.rs	1	99.32%

Totals
Change from base Build 3420272719:	0.1%
Covered Lines:	39529
Relevant Lines:	40132

---

💛 - Coveralls

[panarch]

to_owned() for Copy impl value might be.. too much.
you can simply use *nullable for these cases.

[panarch]

This is also the meaningful improvement.
Looks so so good.
Thanks a lot 👍