feat: support iteration generation

[Blaok]

It'll be convenient to support generating iterative applications automatically. Currently iterative applications have to be manually fused/hard coded.

• [x] Update the grammar to support iteration semantics
• [x] Update the compiler so that it understands iteration semantics

[Blaok]

Some specific issues:

• Iterative stencil needs border preserving
  • [x] Need to forward tensor borders properly
  • [x] Need to forward tile borders properly
  • [x] Need to specify where to forward the borders
• Forwarding status is controlled by aux parameters (i,j,k,l,p,q,r)
  • [x] Need to fix aux params
• [x] Need to handle store indices properly
  • [x] Need to normalize indices

[Blaok]

To better support iterative semantic, an iterate: INT keyword shall be added in the grammar, which means the whole pipeline, from input to output, shall be repeated INT times.

Iterate keyword shall trigger border preserving automatically. Iterate keyword is mandatory. Border preserving need to be trigger by the border keyword border: preserve. Non-iterative algorithm shall specify iterate: 1.

[Blaok]

Currently only support preserve input to output. Iterative algorithm will preserve border from input to output in each iteration.

[Blaok]

Solved completely by #19 (support tiled iterative algorithms).