Adjust for Gambit and update tests

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Gambit doesn't display integer part of numbers
whose absolute value is 0 < x < 1.

I adjusted the code of 'compose-with-digits' for such case.

Also, I updated tests and added test-Gambit.scm and test-Guile.scm.

I confirmed that all tests are passed.

OS: Windows 8.1 (64bit)
DevTool: MSYS2/MinGW-w64 (64bit) (gcc version 7.1.0 (Rev2, Built by MSYS2
project))
Scheme: Gauche v0.9.6_pre4, Sagittarius v0.8.4, Racket v6.7
        Gambit v4.8.8, Guile v2.0.14

I created a pull request as follows.
https://github.com/scheme-requests-for-implementation/srfi-48/pull/5

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<div dir=3D"ltr"><div>Gambit doesn&#39;t display integer part of numbers</d=
iv><div>whose absolute value is 0 &lt; x &lt; 1.</div><div><br></div><div>I=
 adjusted the code of &#39;compose-with-digits&#39; for such case.</div><di=
v><br></div><div>Also, I updated tests and added test-Gambit.scm and test-G=
uile.scm.</div><div><br></div><div>I confirmed that all tests are passed.</=
div><div><br></div><div>OS: Windows 8.1 (64bit)</div><div>DevTool: MSYS2/Mi=
nGW-w64 (64bit) (gcc version 7.1.0 (Rev2, Built by MSYS2 project))</div><di=
v>Scheme: Gauche v0.9.6_pre4, Sagittarius v0.8.4, Racket v6.7</div><div>=C2=
=A0 =C2=A0 =C2=A0 =C2=A0 Gambit v4.8.8, Guile v2.0.14</div><div><br></div><=
div><div>I created a pull request as follows.</div><div><a href=3D"https://=
github.com/scheme-requests-for-implementation/srfi-48/pull/5">https://githu=
b.com/scheme-requests-for-implementation/srfi-48/pull/5</a></div></div><div=
><br></div></div>

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