New function to retrieve coltypes

[rousbound]

I stumbled across the need to use the getcoltypes and getcolnames from luasql and thought the following function could live on dado:

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-- Retrieves types and order of columns
-- @param self Dado Object.
-- @param columns String with fields list.
-- @param tabname String with table name.
-- @param cond String with where-clause (and following SQL text).
-- @param mode String indicating fetch mode (default == 'a').
-- @see dado.sql.select
-- @return Table with the entire result set.
function M.coltypes(self, columns, tabname, mode)
	mode = mode or "a"
	columns = columns or "*"
	local query = sql.select(columns, tabname, nil, "limit 1")
	local cur = self:cursor(query)
	local colorder = cur:getcolnames()
	local types = cur:getcoltypes()
	local t = {}
	if mode == "a" then
		for i,col in ipairs(colorder) do
			t[col] = types[i]
		end
	elseif mode == "n" then
		for i,col in ipairs(colorder) do
			t[i] = {
				col = v
			}
		end
	end
	return t
end

Use case:

local rs = conn:coltypes("*", "user", "a")
print(table.tostring(rs))
{
    id_user = "integer",
    admin = "bool"
    (...)
}

local rs = conn:coltypes("*", "user", "n")
print(table.tostring(rs))
{
   [1] = { col = "id", type ="integer" },
   [2] = { col = "admin", type = "bool" }
   (...)
}