Fix block scoped `const`.

• Test both strict and non-strict modes (nodent-compiler)
• Avoid deprecation warning in NodeJS 10 for new Buffer()
• Fix regression in 3.2.5 for var declared in loop bodies
• Treat const as block scoped by default (pre-ES6 implementations usually treated const as function scoped). See MatAtBread/nodent-transform@5fa3d01