WIP: Smarter fusion

This adds two important (for me) control points to fuse_kernels.

1. This allows for initialized const. temporary variables to not be duplicated when fusing kernels (if they are identically defined between kernels).
2. This allows duplicate instructions (specified by ID) to not be duplicated when fusing multiple kernels that contain them.

The use case here is that I need both the temporary variables and the instructions in subkernels for unit testing. However, when I smush all the subkernels together into a global kernel they shouldn't be duplicated for efficiency.

As a simple example

import loopy as lp
import pyopencl as cl
import numpy as np
from loopy.kernel.data import temp_var_scope

lp.set_caching_enabled(False)
ctx = cl.create_some_context(interactive=False)
queue = cl.CommandQueue(ctx)

knl1 = lp.make_kernel('{[i]: 0 <=i < 10}',
"""
<> mid = data[i] < 5 {id=mid}
out1[i] = 10 * mid
""",
[lp.TemporaryVariable('data', initializer=np.arange(10), scope=temp_var_scope.GLOBAL, read_only=True,
shape=(10,)),
'...'])

knl2 = lp.make_kernel('{[i]: 0 <=i < 10}',
"""
<> mid = data[i] < 5 {id=mid}
out2[i] = 10 * mid
""",
[lp.TemporaryVariable('data', initializer=np.arange(10), scope=temp_var_scope.GLOBAL, read_only=True,
shape=(10,)),
'...'])

knl = lp.fuse_kernels([knl1, knl2], duplicate_intialized=False, collapse_insns_ids=['mid'])

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

results in

#define lid(N) ((int) get_local_id(N))
#define gid(N) ((int) get_group_id(N))

__constant long const data[10] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };

__kernel void __attribute__ ((reqd_work_group_size(1, 1, 1))) loopy_kernel_and_loopy_kernel(__global int *restrict out1, __global int *restrict out2)
{
  int mid;

  for (int i = 0; i <= 9; ++i)
  {
    mid = data[i] < 5;
    out2[i] = 10 * mid;
    out1[i] = 10 * mid;
  }
}

todo:

• Note, this requires the merge request I pushed to pymbolic to work (I don't know how to make loopy CI use that yet!)
• There's still a few bugs cropping up in the instruction checking I need to fix