Remove keywords *begin, *end and *then

[Daniel-Cortez]

What this PR does / why we need it:

Removes keywords *begin, *end and *then, as well as alternative syntaxes for if, switch, for, while and do-while statements, as explained in #611.

Which issue(s) this PR fixes:

Fixes #611

What kind of pull this is:

• A Bug Fix
• A New Feature
• Some repository meta (documentation, etc)
• Other

Additional Documentation:
This PR also includes the changes from #680 (they are needed to prevent a crash in one of the tests), so you'll probably want to review and merge that PR first.

[stale]

This issue has been automatically marked as stale because it has not had recent activity.

[Y-Less]

Merged locally.

[Daniel-Cortez]

While working on this PR, I noticed one interesting detail:

main()
{
    new x = 0;
    do { ++x; } while x < 10; // this would compile, even though
                              // there are no parentheses around 'x < 10'
}

Apparently, the parentheses around the control expressions of do..while loops are optional. But for some reason they're only optional for do-while loops; in all other control flow statements (while, for, if, switch) parentheses are mandatory.
I'm not sure why there's this strange rule for do..while statements, there are no comments about this syntax in the compiler code, and the Language Guide doesn't seem to say anything about this either.

I wonder if this syntax is already used somewhere. Because if not, then removing it would allow to greatly simplify the code in function test() (sc1.c). This PR is about removing excess syntax anyway.