Normalize block delimiters

[ghost]

By the end of the preparsing stage, all 'do'-enclosed blocks of the input source code should be converted into bracket-enclosed ones. For instance, the following code snippet:

coll.each do |l|
    puts l
end

Should be transformed into:

coll.each {|l| puts l}

Such transformation would greatly reduce the parser's work while inspecting blocks.

[ghost]

Since there are optional do's possibly introduced by both 'until' and 'while' keywords, and these do's don't introduce additional 'end' keywords, we need a way to differ between such do's and the ones that do introduce the 'end' keyword - there appears to be no easy way of doing that without context, due to cases such as the following:

until (coll.each do end)
end

This block is semantically equivalent to the one below:

until (coll.each do end) do
end