Introduces a SubscriptOp that allows to write IR like
```
func.func @load_store(%arg0: !emitc.array<4x8xf32>, %arg1: !emitc.array<3x5xf32>, %arg2: index, %arg3: index) {
%0 = emitc.subscript %arg0[%arg2, %arg3] : <4x8xf32>, index, index
%1 = emitc.subscript %arg1[%arg2, %arg3] : <3x5xf32>, index, index
emitc.assign %0 : f32 to %1 : f32
return
}
```
which gets translated into the C++ code
```
v1[v2][v3] = v0[v1][v2];
```
To make this happen, this
- adds the SubscriptOp
- allows the subscript op as rhs of emitc.assign
- updates the emitter to print SubscriptOps
The emitter prints emitc.subscript in a delayed fashing to allow it
being used as lvalue.
I.e. while processing
```
%0 = emitc.subscript %arg0[%arg2, %arg3] : <4x8xf32>, index, index
```
it will not emit any text, but record in the `valueMapper` that the name
for `%0` is `v0[v1][v2]`, see `CppEmitter::getSubscriptName`. Only when
that result is then used (here in `emitc.assign`), that name is inserted
into the text.
