implemented const_cast as a macro

[badasahog]

I changed const_cast to be a type agnostic macro, much more in line with the C++ version.

[github-actions]

• HEAD=constCastMacro stopped early for DT[,.SD] improved in #4501
  

Generated via commit 49ddf38

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	5 minutes and 3 seconds
Installing different package versions	9 minutes and 41 seconds
Running and plotting the test cases	2 minutes and 33 seconds

[aitap]

This seems to crash fread. We only have one use for casting away the const: to fiddle with the memory-mapped file. There is no need for the generic version. If you would like to get rid of union type punning, let's launder the non-const pointer through uintptr_t. If you have time, try to find out which compiler emits warnings when casting a const char * to char * - what if we don't have to worry about it?

[Anirban166]

In the issue titled 'awful one liners don't improve readability or performance' (#6995) I see you wrote:

Collapsing everything into one line doesn't make the code run faster, the compiler is smart enough to sort it out.

I wonder what category this belongs to then 🤔

[badasahog]

A fair criticism!

The problem is, I don't think it's even possible to implement it differently since its a macro, and I think const_cast already has a clear enough meaning that the one liner isn't too bad here. It's clear what the code does.