Add postgresql prepared queries


- make use of PQexecParams to pass parameters separately from the SQL command text (recommanded against SQL injection - stop relying on string formatting for sql parameter passing and use the postgres API)

- add postgresql prepared queries:

```nimrod
# open first the db connection with db = open(...), then:

const query = sql"SELECT id, randomNumber FROM World WHERE id = $1"
var my_prepared_query: TSqlPrepared

# do this once
my_prepared_query = db.prepare("world", query, 1)  # the query has 1 parameter

# usage (put the following in your request handlers, for example):
let row = db.getRow(my_prepared_query, random(10_000))
```