Make built-in SQL functions and operators available in @funsql macro

[xitology]

For use with @funsql macro, FunSQL currently exports a small number of common SQL functions and operators:

• https://github.com/MechanicalRabbit/FunSQL.jl/blob/master/src/nodes/function.jl#L202
• https://github.com/MechanicalRabbit/FunSQL.jl/blob/master/src/nodes/aggregate.jl#L175

Any SQL function not in these lists has to be declared like this:

const var"funsql_%" = FunSQL.Fun."%"
const funsql_array_get = FunSQL.Fun."?[?]"
const funsql_as_integer = FunSQL.Fun."(?::integer)"
const funsql_bool_and = FunSQL.Agg.bool_and
const funsql_regexp_matches = FunSQL.Fun.regexp_matches
const funsql_string_agg = FunSQL.Agg."string_agg(?, NULL ORDER BY ?)"

Ideally, FunSQL should provide all built-in functions and operators, for every supported database engine. Since every engine has its own list of supported functions, these declarations could be added to engine-specific modules:

using FunSQL, FunSQL.PostgreSQL
@funsql select(as_integer("0")) #-> SELECT ('0'::integer)