…onversions

Retrieve the underlying types to perform smarter conversions.
More importantly, this change intelligently flags overflowing
conversions for homogenous signedness aka:
* int* -> int*
* uint* -> uint*

whereby for each same signedness, just check widths where:
+ 64-bit machines:
uint64 == uint > uint32 > uint16 > uint8
int64  == int  > int32  > int16  > int8

+ 32-bit machines:
uint64 > uint == uint32 > uint16 > uint8
int64  > int  == int32  > int16  > int8

and this change only flags the offending non-fitting conversions.
For an exhibit, this code
```go
package inttests

type in = int
type uin = uint

func it() {
	_ = uint64(uint32(0))
	_ = uint(uint32(0))
	_ = uint(uint16(0))
	_ = uint(uint8(0))
	_ = uint(uint64(0))
	_ = uint32(uint64(0))
	_ = uint16(uint64(0))
	_ = uint8(uint64(0))
	_ = uint8(uint(0))
	_ = uint8(uint32(0))
	_ = uint8(uint64(0))
	_ = int(uint(0))
	_ = in(uint(0))
	_ = uin(uint(0))
	_ = uin(uint32(0))
}
```

* Previously:
+ Could not catch the aliased int with overflowing potential from uint
+ Falsely flagged all the rest as overflowing so 12 issues

* Currently:
+ Catches the aliased int with overflowing potential from uint
+ Only flags the actually overflowing conversions so only 8 issues

Fixes #56
Fixes #57