schema for schemas

Following the chat today with @TheCedarPrince and @quffaro, we may like to update the schema for schemas to handle compound PKs (and FKs). I have been thinking about the following one:

```julia
using Catlab

"""
A schema to describe SQL schemas.
Tables are what you would expect. Cols are the columns of a table, so they always know who they belong to col_of.
PK is the primary key of a table pk_of. It needs a junction table PK_Cols because composite primary keys need
to reference multiple columns of a table. FK are foreign keys, they take you from one table to a PK (of another table).
Because the referenced primary key may be composite, the foreign key may also need to be composite, so there is a junction
table FK_Cols to relate a foreign key to its columns.

There are 2 commuting squares. The first is defined by pk_col; col_of == pk; pk_of, saying that a primary key's columns
must belong to the table that it is a primary key for. The second is analogous for foreign keys and is defined by fk; from == fk_col; col_of.
We would also like to say that from != to; pk_of so that foreign keys have to go between different tables but I do not know
how to enforce that.
"""
@present SchSqlTables(FreeSchema) begin
(Tables,Cols,PK,PK_Cols,FK,FK_Cols)::Ob
col_of::Hom(Cols,Tables)
pk_col::Hom(PK_Cols,Cols)
pk::Hom(PK_Cols,PK)
pk_of::Hom(PK,Tables)
to::Hom(FK,PK)
from::Hom(FK,Tables)
fk::Hom(FK_Cols,FK)
fk_col::Hom(FK_Cols,Cols)
StrType::AttrType
tab_name::Attr(Tables,StrType)
col_name::Attr(Cols,StrType)
col_type::Attr(Cols,StrType)
compose(pk, pk_of) == compose(pk_col, col_of)
compose(fk_col, col_of) == compose(fk, from)
# compose(to, pk_of) != from
end

to_graphviz(SchSqlTables, graph_attrs=Dict(:size=>"5", :ratio=>"expand"))
```


Which would feed into a nice visualization, which could at first be made just by directly building up the Graphviz syntax:

```
strict digraph {
rankdir="LR"
node [shape="plain"]

TableA [label=<

TableA
🔑id
🔗tab_b

>];

TableB [label=<

TableB
🔑name

>];

TableA:tab_b:e -> TableB:name:w
}
```

See visualization at https://magjac.com/graphviz-visual-editor/?dot=strict%20digraph%20%7B%0Arankdir%3D%22LR%22%0Anode%20%5Bshape%3D%22plain%22%5D%0A%0ATableA%20%5Blabel%3D%3C%0A%20%20%20%20%3CTABLE%20BORDER%3D%220%22%20CELLBORDER%3D%221%22%20CELLSPACING%3D%220%22%3E%0A%20%20%20%20%20%20%20%20%3CTR%3E%3CTD%20COLSPAN%3D%222%22%3ETableA%3C%2FTD%3E%3C%2FTR%3E%0A%20%20%20%20%20%20%20%20%3CTR%3E%3CTD%3E%F0%9F%94%91%3C%2FTD%3E%3CTD%3Eid%3C%2FTD%3E%3C%2FTR%3E%0A%20%20%20%20%20%20%20%20%3CTR%3E%3CTD%3E%F0%9F%94%97%3C%2FTD%3E%3CTD%20PORT%3D%22tab_b%22%3Etab_b%3C%2FTD%3E%3C%2FTR%3E%0A%20%20%20%20%3C%2FTABLE%3E%0A%3E%5D%3B%0A%0ATableB%20%5Blabel%3D%3C%0A%20%20%20%20%3CTABLE%20BORDER%3D%220%22%20CELLBORDER%3D%221%22%20CELLSPACING%3D%220%22%3E%0A%20%20%20%20%20%20%20%20%3CTR%3E%3CTD%20COLSPAN%3D%222%22%3ETableB%3C%2FTD%3E%3C%2FTR%3E%0A%20%20%20%20%20%20%20%20%3CTR%3E%3CTD%20PORT%3D%22name%22%3E%F0%9F%94%91%3C%2FTD%3E%3CTD%3Ename%3C%2FTD%3E%3C%2FTR%3E%0A%20%20%20%20%3C%2FTABLE%3E%0A%3E%5D%3B%0A%0ATableA%3Atab_b%3Ae%20-%3E%20TableB%3Aname%3Aw%0A%7D