Use 'unsafe' to modify private struct fields, allowing to keep
unmodified field names: before they were prefixed with a capital.

Parse package statement. Provide a also a default package name for
REPL and tests. The support of packages is still incomplete.

* fix (parser): don't panic if assign of define untyped value

In case of defining or assigning to untyped value, the type has
to be taken from the source value instead of the target value.

The vm test coverage has also been slightly improved.

* fix and simplify Token.Name()

* improve parser errors

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.

Also detection and automatic execution of main function.
Make sure that all debug is output to stderr.

Fix: small README fixes and throw an error if expression not supported.

Co-authored-by: Marc Vertes <mvertes@free.fr>

Signed-off-by: Antonio Navarro Perez <antnavper@gmail.com>

Type and Value types in vm package are now used in place of reflect.Type
and reflect.Value. It allows to remove the dependency on reflect for
parser and compiler packages.

The main purpose of Type is to provide a solution to implement recursive
structs, named types, interfaces and methods, despite the limitations of
Go reflect. The goal is to provide the thinnest layer around reflect.