adding mruby-string-utf8 breaks String#[float]

fix ISBLANK() for Visual Studio < 2013

Visual Studio versions older than 2013 lack C99's isblank().
Since only ASCII characters are passed to it, implement it
directly without calling the locale-specific isblank().

adhoc fix for pass build

Fix: Numeric#step infinite loop.

mrdb: add missing null-termination

mrdb: fix two memory leaks

mrdb: minor changes

remove unnecessary _WIN32 preprocessor check

SIZE_MAX < UINT32_MAX is false on Win32 / Win64.

Fix String#slice! raise TypeError or return invalid value.

use mrb_str_xxx_lit for mrdb.

mrdb: fix heap-use-after-free

clang 3.5.0 with -Wextra produces a -Wignored-qualifiers diagnostic.

C99+TC3, 7.19.7.1p2: [...] the fgetc function obtains that character as
an unsigned char converted to an int [...]