fix regression with cache

[jcjaskula-aws]

This change modified the cache objects. We were not caching the ast nodes anymore but the ouput of to_cached_oqpy_expression. This led to regression in benchmarks.

Solution: we store the ast in Program.expr_cache. I changed to_cached_oqpy_expression return type to be AstConvertible to match to_oqpy_expression. Consequently, we need to call the main to_ast function instead of HasToAst.to_ast.

Updated a unit test.

[PhilReinhold]

Looks good to me, but this could have been caught by mypy I think. The problem is that currently our lock file has openpulse=0.5.0 which does not export typing. We should try to fix this at some point by updating openpulse in our lock file and fixing the resulting mypy errors. We should probably also add a py.typed file so that downstream packages can use oqpy's type hints.