v0.1.8: Uniswap works for Solana and Substrate

Added
- Added a strength reduce pass to eliminate 256/128 bit multiply, division,
  and modulo where possible.
- Visual Studio Code extension can download the Solang binary from github
  releases, so the user is not required to download it themselves
- The Solana target now has support for arrays and mapping in contract
  storage
- The Solana target has support for the keccak256(), ripemd160(), and
  sha256() builtin hash functions.
- The Solana target has support for the builtins this and block.timestamp.
- Implement abi.encodePacked() for the ethereum abi encoder
- The Solana target now compiles all contracts to a single `bundle.so` BPF
  program.
- Any unused variables, events, or contract variables are now detected and
  warnings are given, thanks to [LucasSte](https://github.com/hyperledger-labs/solang/pull/429)
- The `immutable` attribute on contract storage variables is now supported.
- The `override` attribute on public contract storage variables is now supported.
- The `unchecked {}` code block is now parsed and supported. Math overflow still
  is unsupported for types larger than 64 bit.
- `assembly {}` blocks are now parsed and give a friendly error message.
- Any variable use before it is given a value is now detected and results in
  a undefined variable diagnostic, thanks to [LucasSte](https://github.com/hyperledger-labs/solang/pull/468)

Changed
- Solang now uses LLVM 12.0, based on the [Solana LLVM tree](https://github.com/solana-labs/llvm-project/)

Fixed
- Fix a number of issues with parsing the uniswap v2 contracts
- ewasm: staticcall() and delegatecall() cannot take value argument
- Fixed array support in the ethereum abi encoder and decoder
- Fixed issues in arithmetic on non-power-of-2 types (e.g. uint112)

Signed-off-by: Sean Young <sean@mess.org>