New compiler: Fix bug where naming a parameter like its function caused a crash

[fernewelten]

Fixes #2647.

Parameters act as variables within the function for which they are defined. This leads to trouble inside the compiler when a parameter is named like its own function. Added code that forbids this.

Typical code:

function foo(int foo, int x) 
{}

I took this as an opportunity to streamline the function defining code in general.

[ivan-mogilko]

I can confirm that this fixes the aforementioned crash. But I'm actually not certain if it is correct to disable parameters of same name, because that does not prevent from using this function outside of function, and then local variables of same name are allowed, so this makes things not consistent.

EDIT: both C# and C++ appear to have no problem with either function parameter or local var of the same name as a function.
To clarify, I don't care about being able to name parameters as a function, but my concern is about parameters and local variables treated consistently here, as these are practically same thing from the function's perspective (I think).