The VM Vassign instruction now takes an argument to indicates the
number of assignations to perform on the stack.
Definitions with a function returning multiple values now work.
There is still some simplifications, and also to apply the same
strategy to var declarations with assign.

Do not consider assign operators in expression handling, they are
part of statements.
Isolate parsing of left hand side and right hand side in assignments.

Handle assign statements in parseAssign function. Detect multi-assign
by the presence of comma in the right hand side tokens.
The case of swap is not handled yet.

Branching control is delegated directly to the Next instruction,
which now takes the location of loop exit as first argument.
It avoids the use of JumpFalse, plus the stack storage for the
condition.

- 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.