Merge pull request #5084 from mruby/mruby3

Mruby3

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+User visible changes in `mruby3`
+===
+
+# Build System
+
+You can specify `TARGET` option to `rake` via a command line
+option, or via an environment variable, e.g.
+
+`rake TARGET=host all`
+
+or
+
+`TARGET=host rake all`
+
+It's much easier to switch multiple targets than the
+previous `build_config.rb` system.
+
+## `presym` target
+
+The first compilation of `mruby` may require generation of a
+static symbol table named `build/presym`. You can generate
+the table by `rake gensym`.
+
+## `target` directory
+
+Build target specification files are loaded from `target`
+directory. The default `TARGET` is `host` which is described
+in `target/host.rb`. There are many other targets for example:
+
+* `host-gprof`: compiles with `gprof` for performance tuning
+* `host-m32`: compiles in gcc 32bit mode on 64bit platforms
+* `boxing`: compiles all three boxing options
+* `clang-asan`: compiles with `clang`'s Address Sanitizer
+
+`target/host.rb` comes  with comments to help  writing a new
+target description.
+
+If you want to have your target description out of the
+source tree, you can specify the path to the description
+file in `MRUBY_CONFIG`.
+
+# Build Target Contribution
+
+When you write a new target description, please
+contribute. We welcome your contribution as a GitHub
+pull-request.
+
+# Languge Changes
+
+## New Syntax
+
+We have ported some new syntax from CRuby.
+
+* R-assignment (`12 => x`)
+* Numbered block parameter (`x.map{_1 * 2}`)
+* End-less `def` (`def double(x) = x*2`)
+
+# Configuration Options Changed
+
+Some configuration macro names are changed for consistency
+
+## `MRB_NO_FLOAT`
+
+Changed from `MRB_WITHOUT_FLOAT` to conform `USE_XXX` naming
+convention.
+
+## `MRB_USE_FLOAT32`
+
+Changed from `MRB_USE_FLOAT` to make sure `float` here means
+using single precision float, and not the opposite of
+`MRB_NO_FLOAT`.
+
+## `MRB_USE_METHOD_T_STRUCT`
+
+Changed from `MRB_METHOD_T_STRUCT`.
+
+To use `struct` version of `mrb_method_t`. More portable but consumes more memory.
+Turned on by default on 32bit platforms.
+
+## `MRB_NO_BOXING`
+
+Uses `struct` to represent `mrb_value`. Consumes more memory
+but easier to inveticate the internal and to debug. It used
+to be default `mrb_value` representation. Now the default is
+`MRB_WORD_BOXING`.
+
+## `MRB_WORD_BOXING`
+
+Pack `mrb_value` in an `intptr_t` integer. Consumes less
+memory compared to `MRB_NO_BOXING` especially on 32 bit
+platforms. `Fixnum` size is 31 bits so some integer values
+does not fit in `Fixnum` integers.
+
+## `MRB_NAN_BOXING`
+
+Pack `mrb_value` in a floating pointer number. Nothing
+changed from previous versions.
+
+## `MRB_USE_MALLOC_TRIM`
+
+Call `malloc_trim(0)` from mrb_full_gc() if this macro is defined.
+If you are using glibc malloc, this macro could reduce memory consumption.
+
+# Internal Changes
+
+## `Random` now use `xoshiro128++`.
+
+For better and faster random number generation.