Downloading binaries from `emscripten-asmjs/` and `emscripten-wasm/` dirs in solc-bin

[cameel]

We introduced the new directory structure in solc-bin quite a while ago and we're recommending it in Static Binaries. solc-js, however, still uses the legacy bin/ directory. We should switch to the new structure.

• Make downloadCurrentVersion.ts use the new dirs.
  • It should try emscripten-wasm/ first and only fall back to emscripten-asmjs/ when the binary is not there.
  • Nightlies are still only available in bin/. We need to resolve Add pre-release/nightly builds inside the list of static binaries solidity#13015 first.
• Find any other places where we're using the old dirs and fix them too.
• Fix all references to the old dirs in the README.

For reference, Truffle has recently switched to the new structure too: ConsenSys-archive/truffle#5008.