A new `Composite` token is created. Literal composite expressions
are recognized and partially handled by the parser but not yet
by the code generator.

Other cosmetic changes are present.

* 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.

A 'New' instruction is added in VM to manage initialisation
of typed variables in the stack. The instantiated type symbols
are now added to global data. Accessing and setting values by
address is now working.

Recursive structs and embedded structs are not supported yet.

Only symbols are produced, no bytecode is emitted. The constant
expressions are evaluated at compile time using the stdlib package
go/constant. The parser handles implicit repetition of the last
non-empty expression list. The iota symbol is reset to 0 and
incremented for each line of a const block.

To be done in a next commit: type conversions.