function parameters should be assigned in same block as function body

[jpacik]

The current implementation of a Flux function expression puts the function body in a separate block nested inside the enclosing block where the function parameters are declared. This means that a function's parameters can effectively be reassigned in its own body (see #851). However function parameters need to be immutable inside the function body.

This issue is for refactoring the implementation of Flux function expressions by placing the parameter declarations in the same block as the function body.