refactoring of SiPLE example

[christoff-buerger]

The SiPLE example is not consistent with the other examples; best practices of RACR-based languages are not adapted yet, in particular the use of specialised child and attribute accessors like (->foo n) to query the foo child of n or (=bar n) to query the bar attribute of n. Refactoring to update it respectively is required.

[christoff-buerger]

The issue is related to issue #43.

[christoff-buerger]

For documentation reasons it may be better to change SiPLE to an Oberon compiler/interpreter. This could be done throughout the refactoring process and requires also the refactoring of the parser (cf. issue #43).

However, the priority of a change to Oberon and the refactoring is low.