Fixes#1077. This was caused by the assumption that a block
cursor is represented as zero width internally and simply
rendered to be a single width selection, where as in reality
a block cursor is an actual single width selection in form and
function.
Behavioural changes:
1. Surround selection no longer works when cursor is _on_ a
surround character that has matching pairs (like `'`
or `"`). This was the intended behaviour from the start
but worked till now because of the cursor position
calculation mismatch.
For example when the cursor is _on_ the `'` in `'word'`, the cursor
wouldn't move because the search for a matching pair started _from_ the
position of the cursor and simply found itself.
Surround operations previously ignored other pairs that are
enclosed within which should be skipped. For example if the
cursor is on the `,` in `{{a},{b}}`, doing `md{` previously
would delete the `{` on the left of `a` and `}` on the right
of `b` instead of the outermost braces. This commit corrects
this behavior.
