* chore: refactor to keep only the new parser and bytecode vm

* scanner: remove Token.value field

* scanner: remove scanner.kind field

* chore: move language specification in lang package

This avoid a cyclic dependency in scanner_test which can now use
the golang/GoSpec language specification for Go.

* clean code

* scanner: export scanner fields

Also parser now generate function calls, including externals.

* chore: fix lint issues

* parser: handle strings

* wip

* parser: implement support for 'if, else, else if' statements

Resolving labels in the compiler still in progress.

* parser: support if statements, improve compiler

* improve handling of functions

* improve support of local variables

* scanner: trim leading and trailing spaces

* fixes to make fibonacci work

* parser: improve README, fix function parameters parsing

The scanner returns a slice of pointers to tokens instead of a
slice of tokens. The parser now pass the initial node context.

Refctor node kind names by concatenating category and instance, to
allow better sorting. Comments are now parsed and skipped during
generation of AST.

So multiple successive incremental Evals function correctly.

Also improve the following:
- Apply the same Eval API to vm0 and vm1
- parser: dot diagram display is now synchronous
- codegen: outsource complex code generation for readability
- vm1: Pop take the number of values to pop as operand

This makes the code easier to use.

* parser: define all node kinds to make the parser multi-language

Defining all AST node kinds in the parser is necessary to make the
parser really multi-language. If a language requires a node kind not
already present in parser/kind.go, it will be necessary to add it first
here.

Note that as long as a node kind subtree is structurally identical
between languages, even if there are lexical and/or syntaxic
differences, it can (and must) be shared amongst multiple language
definitions.

For example, an "if" statememt in shell script or in C code should give
the same `IfStmt` at AST level.

In order to let the parser deal with the various language syntaxes,
and produce the right node kind and subtree, parser flags will be set
in language definitions (see `Flags` field in `NodeSpec` struct).

* lang/golang: use parser node kinds

* vm0: remode dependency on language definition.