[Uim] Replacing some preedit relevant codes with 'ustr'

[YamaKen]

Hi Hiroyuki, Takuro,

I'm going to replace some complicated preedit relevant codes
with 'ustr' I wrote recently. Although I believe that there is
no problem, stop me immediately if you feel some problem with
its design or the meaning of the name.

'ustr' stands for 'universal (editable) string'. It represents a
logical order string of abstract elements for general
purpose. Since it does not assume any specific type of the
elements, we can reuse it against several generations of
composition table architectures such as rk or hk. It should be
isolated from composition table architecture to keep
flexibility, reusability and simplicity. ustr provides only
basic string and editing operations.

Since ustr is not dedicated to implement preedit, I named it
generic one rather than 'preedit'.

See test/test-ustr.scm to understand how it works.

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YamaKen  yamaken at bp.iij4u.or.jp