Converted grammar markup from Confuence wiki to Markdown

Please check the following:

`toplevel.grm`
* lines 4, 31, 51 - link to nonexistent page

`types.grm`
* line 4 - link to right page or not?
* line 17 - added semicolon to the end of block
* line 51 - removed comment

`expressions.grm`
* line 4 - link to nonexistent page

`when.grm`
* line 4 - link to nonexistent page

`lexical.grm`
* line 61 - link to non implemented feature Non-local returns