Add Expr::asterisk() and Expr::tbl_asterisk(table: DynIden) methods - Fix #217

[RomainMazB]

This fix #217, adding two new methods:

• Expr::wildcard()

Query::select()
    .expr(Expr::wildcard())
    .from(Char::Table)
    .to_string(MysqlQueryBuilder);

SELECT *
FROM `character`

• Expr::tbl_wildcard(table: DynIden)

Query::select()
    .expr(Expr::tbl_wildcard(Char::Table))
    .column((Font::Table, Font::Name))
    .from(Char::Table)
    .inner_join(Font::Table, Expr::tbl(Char::Table, Char::FontId).equals(Font::Table, Font::Id))
    .to_string(MysqlQueryBuilder);

SELECT `character`.*, `font`.`name`
FROM `character`
INNER JOIN `font` ON `character`.`font_id` = `font`.`id`

[tyt2y3]

@billy1624 how will this affect SeaORM / SeaSchema?

[billy1624]

No impact for SeaSchema. But this part of code has to be updated in SeaORM.
https://github.com/SeaQL/sea-orm/blob/82bad4a376efb875ac32ab601ac2cd9cce191e3f/src/query/combine.rs#L45-L62

[tyt2y3]

Thank you for the contribution! Looks good all-in-all.

Sorry for the name argument. * has not got an official name in SQL, but I am pretty sure it is not called 'wildcard' (which is %), perhaps it could be named Star or Asterisk?

[RomainMazB]

Yeah, no problem for the name: I call * wildcard because this is how it's called in filesystem paths libraries like globset.

I'll rename all the appellation wildcard to asterisk because even if star is shorter, I feel uncomfortable with tbl_star() method

[tyt2y3]

Thank you