Enhance: More operation code/syntax changes

[Mikael Patel]

Time to move forward with the function definition syntax:

`fun { code-block } ; 

And function call:

`fun: 

Some other minor changes:

c ( x1..xn n -- )   drop n elements
C ( x1..xn -- )  clear stack

Last, extension is using "_" prefix only.

[Mikael Patel]

Please see commit 125181f.