std.numeric: clean import & CustomFloat printing fix

Blocker for #2728 
ping @klickverbot  @quickfur 
Reduced example:
```D
module test2155a;
struct B {}
```
```D
template A()
{
	import test2155a : B;
	void A(B b)
	{
		import std.format;
	}
}	

void foo() {
	import test2155a : B;
	B b;
	A(b);
}
```
```
test2155.d(4): Error: undefined identifier B
```