v0.7.3-beta1

This release contains the accumulation of various small improvements and fixes.

Notable improvements:

• The addition of the function slice_string(s, start, end), which makes working with strings easier. Counting is done with unicode codepoints (same as python, but not the same as JS which uses UTF-16).
• In imperative scripts, you can use %return _rel1, rel2, ... to return multiple relations. However, client support is not available yet.
• Functions starts_with and ends_with now work with bytes.
• In imperative scripts, you can use { ... } as _relname to make an ephemeral relation. This saves quite a lot of typing.
• You can now use base64 strings when putting into a field of Vec. The base64 must decode into bytes that can be interpreted as a C-array of little-endian float32/float64, according to the definition of the Vec.
• Except when using :create or :replace, whether you write => in the field list of mutations is immaterial.
• Parameters: the $var syntax in script denotes named parameters. These should be passed in together with the script when calling the database.
• When doing mutations on stored relations, you can omit the fields if they are exactly the same as the program entry, i.e. ?[x, y, z] := ... :put rel {} is equivalent to ?[x, y, z] := ... :put {x, y, z}
• When using constant relation and only a single row is present, the program entry can be deduced, i.e. ?[] <- [[$x, $y, $z]] is equivalent to ?[x, y, z] <- [[$x, $y, $z]]. This works together with the last item to allow writing ?[] <- [[$x, $y, $z]] :put rel {}
• A new set of mutation operators in addition to :put and :rm: these are :insert, :update, and :delete, and these checks for the existence of rows already in the database with the same semantics as in SQL. When doing :updates, you only need to specify the keys and the fields you want to update.
• Mutations now support a :returning option which, if present, will return the changes applied by the mutation instead of the status code.
• Most sysops such as ::index create can be used in imperative scripts now.
• The standalone server now supports a token relation: https://github.com/cozodb/cozo/blob/cd840e687c5bde9dc3c95968f07bbed1b3669a65/cozo-bin/src/server.rs#L65-L67. Contents in this relation can be used as bearer tokens to access the API remotely.