This fixes#713.
If the block before the statement we want to use introduce var on, had empty
lines these empty lines would also be added between the let-statement and
the current line where the new variable is used.
This fixes that by trimming excess newlines from the start of the indent chunk
and simply adding a single newline (when the chunk had newlines) between the
let-statement and the current statement. If there were no newlines this
matches the previous behaviour.
