Better allocator for wasm (#1460)

* Add Arch Linux installation instructions

* Enable tracing heap size

* Extract heap

* Replace linear allocator with buddy allocator

* Fix test

The purose of this big block is for the tests
to test a failure when the block is too big
though. The improved buddy allocation strategy
results in this block fitting on the heap now.
Hence the increase.

* Get rid of memcpy in to_vec()

* fixup: Style and comments

* fixup: Split Linux instructions by distribution

To prevent misunderstandings of people executing both.

* fixup: Remove unnecessary types and code

* fixup: Make Pointers start from 1, remove some panics, code improvements

* fixup: Return 0 on errors

* fixup: Move loop to separate function

* fixup: Use FnvHashMap instead of HashMap

* fixup: Fix error handling

* fixup: Use current_size() instead of used_size()

* fixup: Fix and document allocation offset

* fixup: Remove unnecessary multiplication

* fixup: Fix comments

* fixup: Remove Arch installation instructions

* Revert "Fix test"

This reverts commit 292c177df8efaa4658293748a13b1ab1c0b76ea8.

* fixup: Remove unused code, improve import

* fixup: Proper alignment

* fixup: Do not use internal constant in public description

* fixup: Add comment regarding invariants

* fixup: Move assertion to compile-time check

substrate/Cargo.lock

@@ -3412,6 +3412,7 @@ dependencies = [
  "assert_matches 1.3.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "byteorder 1.2.7 (registry+https://github.com/rust-lang/crates.io-index)",
  "error-chain 0.12.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "fnv 1.0.6 (registry+https://github.com/rust-lang/crates.io-index)",
  "hex-literal 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "lazy_static 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "log 0.4.6 (registry+https://github.com/rust-lang/crates.io-index)",