Reconsider enabled linters

[rfratto]

We enable the following linters:

- deadcode
- errcheck
- goconst
- gofmt
- goimports
- golint
- gosimple
- ineffassign
- megacheck
- misspell
- structcheck
- unconvert
- unparam
- varcheck
- govet
- unused # new from here.
- interfacer
- typecheck

At least interfacer and megacheck are deprecated and should likely be removed. interfacer in particular has questionable usability and can be prone to false-positives.

The list of enabled linters should be fully re-considered and whittled down to a more appropriate list.

[pracucci]

I checked all the available linters and I would suggest:

• Remove interfacer
• Replace megacheck with staticcheck
• Add prealloc: finds slice declarations that could potentially be preallocated

Any feedback?

[stale]

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