fix: support ConstExpr with functions defined via expr.Function

[Yanhu007]

Fixes #809

Problem

expr.ConstExpr("fn") panics when fn was defined using expr.Function() instead of being a method on the environment struct:

expr.Function("identity", func(params ...any) (any, error) {
    return params[0], nil
}, new(func(any) any))

expr.Compile(`identity(1)`, expr.ConstExpr("identity")) // panic!

This is because ConstExpr only looks in the environment object via runtime.Fetch, not in the Functions table populated by expr.Function().

Fix

Check c.Functions first and extract the appropriate function implementation (Func, Fast, or Safe) before falling back to the environment object lookup.

[antonmedv]

Could you please also add a test to cover this API usage?