[tx] Expand TypedValue to include non-namespaced keywords

[ncalexan]

For simplicity, in #214 I only handled namespaced keywords, like :namespaced/keyword. However, the transactor and query engine can and should handle non-namespaced keywords, like :keyword. This ticket tracks adding support for such keywords.

This ticket is similar to #201, but a little simpler and potentially a little longer, since there are no new :db.type/* definitions and no new ValueType. You'll need to:

1. Add a new TypedValue case, preferably by encapsulating the two types of keywords into one enum Keyword { NamespacedKeyword(...), Keyword(...) };
2. Implement the conversions to and from SQL, using the facts that:

• the SQL TEXT representation of ":namespaced/keyword" and ":keyword" doesn't overlap;
• the value type tag of namespaced and non-namespaced keywords is the same (tag 13)

1. Add tests for conversion around

   mentat/db/tests/value_tests.rs

   Line 25 in 1deed24

   fn test_from_sql_value_pair() {

   .
2. Update the transactor to handle the two types of keywords as required -- start by generalizingto_namespaced_keyword to handle both types of keyword (around

   mentat/db/src/lib.rs

   Line 42 in 1deed24

   pub fn to_namespaced_keyword(s: &str) -> Option<symbols::NamespacedKeyword> {

   )
3. Add tests of the new types in the transactor and potentially in the query engine as well.

[ncalexan]

@jsantell you can probably mentor @victorporof to do this ticket, teaching him some of the bits of the transactor. You might want to make sure #201 is closed first.