- vm: added Pull, Next and Stop instructions, to implement iterators
- lang: add Range, Next and Stop tokens
- parser: handle range clause. Still naive and incomplete.
- comp: generate iterator instructions from range clause.

Work in progress. Only initial support for slices. Many more tests
and combinations needed, but the main pattern is there now.

Added 2 VM instructions, FnewE variant of Fnew with a dereference,
and FieldE, variant of Field with dereference.
It's now possible to pointers in structs and literal composites.

A new token MapAssign is added to implement assignements involving
a map, slice or array, an index key expression and a value expression.

The expressions were parsed from right to left, and it was incorrect and
cumbersome. Now they are processed from left to right, with a simpler
and correct handling of precedence rules.

The vm function call syntax has been changed to set the function
before the input arguments on the stack, as to follow the declaring
order in languages.

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.

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.