What is Loopy's function namespace? (was: Is min a reduction or a 2-arg minimum like OpenCL)

Sorry about the many pull requests / issues today! Been playing around with reductions, and breaking things in all sorts of interesting ways :)

Here's a fun one:

```
import loopy as lp
import pyopencl as cl
lp.set_caching_enabled(False)

ctx = cl.create_some_context()
queue = cl.CommandQueue(ctx)

knl = lp.make_kernel('{[i]: 0 < i < 10}',
"""
out[i] = min(i, 5)
""",
['...'])

print lp.generate_code(knl)[0]
```

Results in:
```
File "test.py", line 14, in
print lp.generate_code(knl)[0]
File "/home/nick/loopy/loopy/codegen/__init__.py", line 523, in generate_code
codegen_result = generate_code_v2(kernel)
File "/home/nick/loopy/loopy/codegen/__init__.py", line 383, in generate_code_v2
kernel = preprocess_kernel(kernel)
File "/home/nick/loopy/loopy/preprocess.py", line 912, in preprocess_kernel
kernel = realize_reduction(kernel, unknown_types_ok=False)
File "/home/nick/loopy/loopy/preprocess.py", line 649, in realize_reduction
new_expressions = (cb_mapper(insn.expression),)
File "/home/nick/.local/lib/python2.7/site-packages/pymbolic/mapper/__init__.py", line 134, in __call__
return method(expr, *args, **kwargs)
File "/home/nick/loopy/loopy/symbolic.py", line 1345, in map_reduction
result = self.callback(expr, self.rec, **kwargs)
File "/home/nick/loopy/loopy/preprocess.py", line 559, in map_reduction
"supposed to reduce over: " + ", ".join(bad_inames))
loopy.diagnostic.LoopyError: reduction used within loop(s) that it was supposed to reduce over: i
```

However,

```
import loopy as lp
import pyopencl as cl
lp.set_caching_enabled(False)

ctx = cl.create_some_context()
queue = cl.CommandQueue(ctx)

knl = lp.make_kernel('{[i]: 0 < i < 10}',
"""
out[i] = abs(-i)
""",
['...'])

print knl(queue)[1]
```

works as expected (both are defined in the opencl function manglers).