mark functions as immutable

[cereum]

When using the output of squids.encode(...)in a GENERATED column, postgres was complaining that the functions needs to be immutable.

-- Example Table
CREATE TABLE public.order(
  id integer PRIMARY KEY GENERATED BY DEFAULT AS IDENTITY,
  external_id text UNIQUE NOT NULL GENERATED ALWAYS AS (sqids.encode(ARRAY[id, 0], 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', 5)) STORED,
  amount numeric NOT NULL,
  name text,
  created_at timestamp with time zone DEFAULT timezone('utc', now()) NOT NULL,
  updated_at timestamp with time zone DEFAULT timezone('utc', now()) NOT NULL
);

From the postgres documentation:

An IMMUTABLE function cannot modify the database and is guaranteed to return the same results given the same arguments forever. This category allows the optimizer to pre-evaluate the function when a query calls it with constant arguments. For example, a query like SELECT ... WHERE x = 2 + 2 can be simplified on sight to SELECT ... WHERE x = 4, because the function underlying the integer addition operator is marked IMMUTABLE.

TL;DR Functions marked as IMMUTABLE always return the same output for the same input and do not depend on any external state. Non-IMMUTABLE functions may depend on database state or have side effects.

Accordingly, I've marked the following functions as follows:

Function	IMMUTABLE
sqids.shuffle	Yes
sqids.checkAlphabet	Yes
sqids.toId	Yes
sqids.toNumber	Yes
sqids.encodeNumbers	Yes
sqids.decode	Yes
sqids.encode (both versions)	Yes
sqids.isBlockedId	No
sqids.defaultBlocklist	No

The tests included with the repo all pass locally on my machine with these changes.