Do not coerce to integer when decoding a float value

When an integral value is expected but a fractional value is found, the
current implementation uses std::num::cast() to coerce to an integer
type, losing the fractional part. This behavior is not desirable because
the number loses precision without notice.

This commit makes it raise ExpectedError when such a situation arises.

[breaking-change]
Always use a decimal point when emitting a float value

JSON doesn't distinguish between integer and float. They are just
numbers. Also, in the current implementation, a fractional number
without the fractional part is encoded without a decimal point.

Thereforce, when the value is decoded, it is first rendered as Json,
either I64 or U64. This reduces type safety, because while the original
intention was to cast the value to float, it can also be casted to
integer.

As a workaround of this problem, this commit makes the encoder always
emit a decimal point even if it is not necessary. If the fractional part
of a float number is zero, ".0" is padded to the end of the result.

[breaking-change]
Code cleanup