Removed assertions that were invalid for encodings with non-code-point encoding code unit sequences.
Removed assertions that were invalid for encodings with non-code-point encoding code unit sequences.

Previously, decode tests asserted relationships between the code unit iterators
underlying a decoded code point and specific positions within the code unit
sequence.  These assertions were invalid for encodings that have non-code-point
encoding code unit sequences, particularly at the beginning of a code unit
sequence.  These asserts have now been removed (and some new ones added).

tahonermann/text_view,tahonermann/text_view-range-v3,tahonermann/text_view-range-v3,tahonermann/text_view