[refactor] Separate SNode read/write kernels into a dedicated class

[k-ye]

Related issue = #2196

Purpose

This is one of the steps to decouple SNode from taichi/program. Because many files inside taichi/ir depend on SNode, they transitively depend on taichi/program as well. taichi/program is very high level, and almost depends on everything in the codebase. So without breaking the tie, it is very hard for us to separate out IR.

Note that this PR alone is not enough (actually, far from enough) for modularizing the IR. I'm thinking of moving out SNode::expr later (we may have to have a similar mapping from SNode to expr). The reason is similar:

1. expr is an Expr, which depends on the expression and the frontend IR
2. Frontend IR depends on taichi/program.

As a short-term goal, I think it will be good enough if we can make ir.h/cpp, statements.h/cpp, snode.h/cpp self contained.

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[yuanming-hu]

Cool! LGTM!

[xumingkuan]

LGTM!