doc/mruby3.1.h: separate mruby3.0 changes and mruby3.1 changes.

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+# User visible changes in `mruby3.1` from `mruby3.0`
+
+# Build System
+
+## `build_config` directory
+
+Several configurations for new platforms are added:
+
+* `cross-mingw-winetest.rb`
+* `cross-mingw.rb`
+* `nintendo_switch.rb`
+* `serenity.rb`
+
+Some new configurations for added for convenience.
+
+* `minimal`: minimal configuration
+* `host-f32`: compiles with `mrb_float` as 32 bit `float`
+* `host-nofloat`: compiles with no float configuration
+
+And `android_arm64-v8a.rb` was renamed to `android_arm64_v8a.rb` for consistency.
+
+# Language Changes
+
+## Keyword Arguments
+
+CRuby3.0 compatible keyword arguments are introduced.
+Keyword arguments are basically separated from ordinal arguments
+
+# Configuration Options Changed
+
+Some configuration macros are available:
+
+* `MRB_WORDBOX_NO_FLOAT_TRUNCATE`: by default, float values are packed in the word if possible, but define this macro to allocate float values in the heap.
+* `MRB_USE_RO_DATA_P_ETEXT`: define this macro if `_etext` is available on your platform.
+* `MRB_NO_DEFAULT_RO_DATA_P`: define this macro to avoid using predefined `mrb_ro_data_p()` function
+
+# Internal Changes
+
+## Reintroduced Instructions
+
+`mruby3.0` removed `OP_EXT1`, `OP_EXT2`, `OP_EXT3` for operand extension. But the operand size limitations was too tight for real-world application.
+`mruby3.1` reintroduces those extension instructions.
+
+## Removed Instructions
+
+`mruby3.1` removed following instructions.
+
+* `OP_LOADL16`
+* `OP_LOADSYM16`
+* `OP_STRING16`
+* `OP_LAMBDA16`
+* `OP_BLOCK16`
+* `OP_METHOD16`
+* `OP_EXEC16`
+
+Those instructions are no longer needed by reintroduction of extension instructions.
+
+* `OP_SENDV`
+* `OP_SENDVB`
+
+Those instructions for method calls with variable number of arguments are no longer needed. They are covered by `OP_SEND` instruction with `n=15`.
+
+## New Instructions
+
+`mruby3.1` introduces following new instructions.
+
+* `OP_GETIDX`: takes 2 operands `a[b]`
+* `OP_SETIDX`: takes 3 operands `a[b]=c`
+* `OP_SSEND`: takes 3 operands `a=self.b(c...)`; see `OP_SEND`
+* `OP_SSENDB`: takes 3 operands `a=self.b(c...){...}`; see `OP_SEND`
+
+### `OP_GETIDX` and `OP_SETIDX`
+
+Execute `obj[int]` and `obj[int] = value` respectively, where `obj` is `string|array|hash`.
+
+### `OP_SSEND` and `OP_SSENDB`
+
+They are similar to `OP_SEND` and `OP_SENDB` respectively. They initialize the `R[a]` by `self` first so that we can skip one `OP_LOADSELF` instruction for each call.
+
+## Changed Instructions
+
+### `OP_SEND` and `OP_SENDB`
+
+Method calling instructions are unified. Now `OP_SEND` and `OP_SENDB` (method call with a block) can support both splat arguments and keyword arguments as well.
+
+The brief description of the instructions:
+
+|`OP_SEND`   | BBB | `R(a) = call(R(a),Syms(b),R(a+1..n),R(a+n+1),R(a+n+2)..nk) c=n|nk<<4`                    |
+|`OP_SENDB`  | BBB | `R(a) = call(R(a),Syms(b),R(a+1..n),R(a+n+1..nk),R(a+n+2..nk),&R(a+n+2*nk+2)) c=n|nk<<4` |
+
+When `n == 15`, the method takes arguments packed in an array. When `nk == 15`, the method takes keyword arguments packed in a hash.
+
+### `OP_ARYPUSH`
+
+Now takes 2 operands and pushes multiple entries to an array.
+
+## `String#hash` now use `FNV1a` algorithm
+
+For better and faster hash values.