Add helper functions for SQL(), Pos(), End()

[apstndb]

Contributors need to maintain comments in ast/ast.go and implementations of SQL(), Pos(), and End() methods in ast/sql.go and ast/pos.go.
I believe productability is improved if comments and implementations can be written as similar structure.

I propose to add helper functions to implement the methods.
I believe these functions are useful as long as memefish have ast.Node interface .

Note
These helper functions are already used in PRs(#99, #101, #111).
sqlOpt needs comparable after Go 1.20, so this PR bump Go version to 1.20.
I have refacotored ast.Select, ast.Path and ast.CallExpr to demonstrate the usage and effect of helper functions.

[makenowjust]

I have a comment to lastElem. Other functions are acceptable.

[makenowjust]

I understand your intention, but this function seems too generic. I would not want to add this as it could be used in places it is not supposed to be.

[apstndb]

OK, lastElem can be written as inline, so I will remove it.

(It may be possible to be accepted if T is restricted to Node)

-func lastElem[T any](s []T) T {
+func lastElem[T Node](s []T) T {

[apstndb]

Fixed e1a3e81

[makenowjust]

func lastElemNode[T Node](s []T) T { is acceptable.

[apstndb]

Introduce func lastNode[T Node](s []T) T in 47ea709

[makenowjust]

@apstndb If these PRs are merged, do you rewrite existing implementations with them as well?

[apstndb]

If these PRs are merged, do you rewrite existing implementations with them as well?

If you want to unify style of existing implementation immediately, I can rewrite them.
If not, we will rewrite them as necessary, like the Boy Scout rule.

[apstndb]

I may be better to place functions into sql.go and pos.go rather than util.go.
These functions won't be used in other files, and these files should be self-contained as possible.

[makenowjust]

I may be better to place functions into sql.go and pos.go rather than util.go.

This sounds good 👍

[makenowjust]

LGTM. Thank you!