Reentrant Engine, Rust anonymous functions, in expression, timestamps, bug fixes

[schungx]

This PR does the following:

• call_fn now has only one version and handles tuples of one/zero
• optimize_ast can now control the optimization level
• add in expressions
• add timestamp which is basically std::time::Instant to do rudimentary timing
• deprecates the no_stdlib feature - use Engine::new_raw to create raw Engine
• fix the max call-depth limit so it properly throws a stack overflow error instead of panic-ing
• fix bug in escaping single/double quotes in character/string literals
• Engine is now re-entrant
• support for creating anonymous functions in Rust which transparently calls into an Engine via call_fn
• Error enums are not omitted when certain features are turned on - this is to avoid the application needing to constantly uncomment/comment out match variants when using features.
• supports parsing JSON into an object map
• some more utility functions for arrays and object maps

[jhwgh1968]

Small question: is there a better name than "anonymous functions"?

I always think of the lambda-like functions that rust has:

let x = |f| { f == 0 };
assert!(x(0));

I interpret this as more like "exported functions", where a named function in Rhai gets a function pointer in Rust.

(It might be useful for Rhai to support the syntax above, but that's a separate topic.)

[schungx]

I always think of the lambda-like functions that rust has:

Well, I thought about the name also. However, lambda seems to imply that it somehow captures its environment as a closure, which this is not. Rust's closures (basically lambda's) have a specific meaning.

This is more like a curryed function...

And this is what Rust's documentation says about anonymous functions:

https://doc.rust-lang.org/stable/book/ch13-01-closures.html