Remove inspection function for syntax tree nodes

[chengluyu]

This PR is extracted from #194. But I'm actually quite dubious about this PR, because manually creating a inspect function for every syntax nodes is really troublesome. I wonder if we can do this by macros or is there anything builtin that can achieve the similar results.

TL;DR

The most important change this PR makes is, if you want to:

• see a code representation of a syntax tree, please call showDbg;
• see how a syntax tree is constructed (the raw structure), please call toString.

Changes

• Add showDbg function to Statement (incl. Term, etc), TypingUnit, and Pgrm. The function showDbg was there but not available at all of them.
• Remove toString functions of these syntax nodes, so that the compiler will synthesize a default toString for them.
• Remove mlscript.codegen.Helpers.inspect and replace all its call with toString.

Note

I spent some time to make sure that showDbg behaves in the same way as toString before. As a result, only test files that involves the output from mlscript.codegen.Helpers.inspect should be changed in this PR.

[LPTK]

Nice simplification. But while we're at it, could you please remove the toString printing for all the compiler tests? We should only print this stuff when :p is on. It's far too verbose and noisy to be done by default.

[chengluyu]

Hello, I have gone through it myself, and left two comments on areas I'm not quite sure about.

[chengluyu]

Thanks!