database/sql: allow Scan to work with structs

[moltzaum]

The default behavior of structs is now to sequentially scan the struct fields.
If some of the fields are unexported, the new implementation will defer to the
old behavior. This change does not affect any types that already implement the
Scanner interface.

Testing for embedded struct added.

Updates #22697 (database/sql wishlish)
Fixes jmoiron/sqlx#162

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Message from Gerrit User 5976:

Patch Set 1:

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Message from Gerrit User 12446:

Uploaded patch set 2: Commit message was updated.

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Message from Gerrit User 12446:

Uploaded patch set 3: New patch set was added with same tree, parent, and commit message as Patch Set 2.

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Message from Gerrit User 5080:

Patch Set 3: Code-Review-2

Thank you for this CL. Typically before we add a feature we create an issue "proposal: do such and such", the proposal is accepted, and then code is written.

Sometimes we make initial implementations to go along with the proposal to help see how it would look, even if the initial implementation isn't used.

This adds significant behavior to database/sql. Please open a new issue beginning with "proposal: ...", mention me "@kardianos", and we can talk about the design. In the mean time I'm marking this as won't merge. To be upfront, I would like to do this, but chances are it won't be in this manner.

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This PR is being closed because golang.org/cl/126416 has been abandoned.