Add map literal syntax

[dain]

Description

This PR adds a simple map literal syntax to the grammar. The grammar uses a comma separated list of key/value entries separated by =>. This is similar to the syntax for a function call with named parameters.

Under the covers the literal is translated to map_from_entries.

Examples

map('x' => 1, 'y' => 2);
map(1.1 => 'a', 2.2 => 'b');

Release notes

(X) Release notes are required, with the following suggested text:

## Section
* Add map literal syntax. For example, `map(1.1 => 'a', 2.2 => 'b')` ({issue}`issuenumber`)