Refactor core::char::EscapeDefault and co. structures

Change core::char::{EscapeUnicode, EscapeDefault and EscapeDebug}
structures from using a state machine to computing escaped sequence
upfront and during iteration just going through the characters.

This is arguably simpler since it’s easier to think about having
a buffer and start..end range to iterate over rather than thinking
about a state machine.

This also harmonises implementation of aforementioned iterators and
core::ascii::EscapeDefault struct.  This is done by introducing a new
helper EscapeIterInner struct which holds the buffer and offers simple
methods for iterating over range.

As a side effect, this probably optimises Display implementation for
those types since rather than calling write_char repeatedly, write_str
is invoked once.  On 64-bit platforms, it also reduces size of some of
the structs:

    | Struct                     | Before | After |
    |----------------------------+--------+-------+
    | core::char::EscapeUnicode  |     16 |    12 |
    | core::char::EscapeDefault  |     16 |    12 |
    | core::char::EscapeDebug    |     16 |    16 |

My ulterior motive and reason why I started looking into this is
addition of as_str method to the iterators.  With this change this
will became trivial.  It’s also going to be trivial to implement
DoubleEndedIterator if that’s ever desired.

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