explicit intermediate storage to prevent fusion of long stencil chains

[gysit]

• introduce an operation stops fusion between dependent stencils
• it can be seen as an abstraction of explicit storage

[gysit]

the syntax could look as follows: %0 = stencil.buffer %1 ([0,0,0]:[64,64,60]) : (stencil.temp<?x?x?xf32>) -> stencil.temp<?x?x?xf32>