Added missing vm instructions to allocate a typed value on the stack
and to set a structure field.
It's possible now to generate struct literal composites for non
keyed struct fields.

Pass a language spec as an argument when create a new parser,
compiler or interpreter.

Also move the REPL code in interpreter package.

* chore: add linters and some lint fixes

Configure some golangci-lint linters to get the code quality right.
Apply the first fixes.

Next step will be to add github actions to run lint and tests in
github CI.

* chore: more lint, fixed comments and variable names. no semantic change.

* chore: add Makefile

This makefile is intended to be used as a local substitute to github
actions.