Expose a mechanism to create a new FunctionCallExpr

[jcrevier]

Many of the expression structs have public fields, which enables the AST to be rewritten with them (as a consumer can create them). The FunctionCallExpr struct's fields are marked pub(crate) however. Were these left private intentionally? If yes, would you be open to exposing a way to create this struct (perhaps making any needed validation checks, etc)? If this was an oversight, would you be open to a PR making them public?

I have a use case for rewriting expressions associated with specific fields, and I think without some way to create a FunctionCallExpr, I will not have fully rewritten all expressions in the filter.

[marmeladema]

I am not necessarily against exposing this but other nodes are usually always valid because they are simple constructs.
Functions are much more complicated and its fairly easy to build something manually that is completely invalid.