std.windows.registry exposes too much implementation details

[redstar]

The program

import core.sys.windows.windows;
import std.windows.registry;

void main()
{
auto error = GetLastError();
}

causes an error because GetLastError() is declared and exported from both
modules.

Personally, I do not expect such conflicts from a library. The root cause is
that std.windows.registry exposes too much implementation details.

This pull request changes the following:

• The windows API functions are not exported
• Windows API functions duplicated from core.sys.windows.windows are removed
• Type boolean is replaced by standard bool