[WIP]
Try to make this pass:
```
import Darwin
print(gradient(at: Double(3), in: tan))
print(gradient(at: Double(3), in: Darwin.tan))
print(gradient(at: Double(3), in: Darwin.cos))
```
```
Undefined symbols for architecture x86_64:
"_AD__$s6Darwin3cosyS2dF_PSRS", referenced from:
_main in tgmath-e5045b.o
ld: symbol(s) not found for architecture x86_64
<unknown>:0: error: link command failed with exit code 1 (use -v to see invocation)
```
