feat!: Indexes for nodes in the AST

Indexes allow associating extra data with nodes in the AST
using an array and then quickly looking up the array to fetch
the extra data.

- Index eidx for expressions
- Index sidx for statements
- Index qidx for queries.

AST nodes are not cloneable. Therefore once a module is created,
it is not possible to accidentally create two nodes with the same
index inadvertently via clone.

AST Cleanup
- Make literal expressions (null, val, number, string etc) also structs
  to match all other expressions
- Merge True and False nodes into a single Bool node.

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

Author: anakrish