WIP: Initial virtual table implementation + generate_series() demo extension

[jussisaurio]

TODO:

• heavy documentation of approach, esp. things where it diverges from sqlite / is hacky / is shit
• fix generate_series() implementation edge cases

Feats:

• adds ability to load virtual table modules from extensions
• adds generate_series() as a demo implementation:

Limbo v0.0.13
Enter ".help" for usage hints.
limbo> .load target/debug/liblimbo_series.dylib
limbo> select u.id, u.first_name from users u join generate_series(1, 9, 2) g ON u.id = g.value;
1|Jamie
3|Tommy
5|Edward
7|Aimee
9|Matthew

generate_series() is sort of a bad example because it is a so-called "eponymous virtual table" i.e. you don't construct it like CREATE VIRTUAL TABLE generate_series USING series(foo) but it is always loaded into the schema when the module is loaded.

i will try to implement another demo impl that actually uses CREATE VIRTUAL TABLE ... , although that might bloat the PR. but it would also validate the direction the current FFI interface is going in.