This reverts commit 15d48efa.

`__ENCODING__' returns the current encoding name (string), unlike
CRuby that returns the encoding object.

Array refactorings and speed improvements

Example benchmark:
$ time build/bench/bin/mruby -e "Array.new(2_000_000){ |i| i }.index{ |i| i == 1_999_999 }"

Before:
real    0m0.934s
user    0m0.922s
sys     0m0.003s

After:
real    0m0.590s
user    0m0.583s
sys     0m0.007s

Fix building with MRB_USE_FLOAT=1

For performance reason. Segmented list consumes slightly less memory
but takes a lot of time especially when there are many slots in the
segment lists (e.g. class constants).

Fix string test with 16-bit integers

Fix MSVC warnings in fmt_fp.c

fix fmt_fp.c(329) : warning C4244: 'return' : conversion from 'ptrdiff_t' to 'int', possible loss of data

fix fmt_fp.c(224) : warning C4244: '=' : conversion from 'ptrdiff_t' to 'int', possible loss of data

fix fmt_fp.c(206) : warning C4244: 'initializing' : conversion from 'ptrdiff_t' to 'int', possible loss of data

fix fmt_fp.c(178) : warning C4244: 'return' : conversion from 'ptrdiff_t' to 'int', possible loss of data

fix fmt_fp.c(123) : warning C4244: 'return' : conversion from 'ptrdiff_t' to 'int', possible loss of data

Fixed register windows of OP_SENDs generated during codegen

To enable method cache, define `MRB_METHOD_CACHE` or
`MRB_METHOD_CACHE_SIZE`. The cache size must be power of 2.
The default cache size is 128.

The measurement:
I measured simple benchmarks found in benchmark/ directory. With
method cache enabled, we gained 6-8% performance improvement, with
97-99% cache hit rate.