Command:
sbt scalafmtAll

Change source and other relevant files to use SPDX license
LICENSE file moved from src/ to ./
Changed license file to refer to this per recommendation
using_spdx_license_list_short_identifiers

WARNING: Tests fail with as of yet undiagnosed error
```
[error] Failed: Total 691, Failed 19, Errors 0, Passed 672, Ignored 15
[error] Failed tests:
[error] 	chiselTests.QueueSpec
[error] 	examples.VendingMachineGeneratorSpec
[error] 	chiselTests.HarnessSpec
[error] 	chiselTests.ConnectSpec
[error] 	chiselTests.aop.SelectSpec
[error] 	chiselTests.PopCountSpec
[error] 	chiselTests.CloneModuleSpec
[error] (Test / test) sbt.TestsFailedException: Tests unsuccessful
[error] Total time: 379 s (06:19), completed Sep 30, 2020 12:38:17 AM
sbt:chisel3>
```

This mechanism is not enabled and should not change the behavior of existing tests
A following PR will deliver a switch that will allow changing the backend.
The reasons for this PR
- Treadle tests run much faster, enabling quicker debugging and CI cycles
- This will help ensure fidelity of Treadle to the Verilator backend

A few tests are marked as verilator only due to black box limitations

Change treadle to a direct dependency

I tried to make it a test only dependency but the TesterDriver sits in src/main requiring that
regular compile have access to treadle

Oops, made treadle the default

A number of changes in response to @ducky64 review
- made backend check clearer and add error handling for multiple backends specified
- Fixed duplicate TargetDirAnnotation uses in Treadle backend
- Cleaned up BlackBox test formatting
- Undid unnecessary debugging changes from Counter
- Undid .gitignore change, that should be on another PR

A number of changes in response to @ducky64 review
- Undid debugging changes made to BitWiseOps

Adds new AsyncReset and "abstract" Reset types. Reset is inferred
in FIRRTL to be either AsyncReset or Bool. The "reset type" of a
register is set by the type of its reset signal:

  val asyncReset: AsyncReset = IO(Input(AsyncReset()))
  val syncReset: Bool = IO(Input(Bool()))
  val abstractReset: Reset = IO(Input(Reset()))

  val asyncReg = withReset(asyncReset) { RegInit(0.U) }
  val syncReg = withReset(syncReset) { RegInit(0.U) }
  val inferredReg = withReset(abstractReset) { RegInit(0.U) }

AsyncReset can be cast to and from Bool. Whereas synchronous reset is
equivalent to a mux in front of a flip-flop and thus can be driven by
logic, asynchronous reset requires that the reset value is a constant.
This is checked in FIRRTL.

Inference of the concrete type of a Reset occurs based on the type the
Reset's drivers. This inference is very simple, it is simple forward propagation
of the type, but it allows for writing blocks and modules that are agnostic
to the reset type. In particular, the implicit `reset` value in MultiIOModule
and thus Module is now concretely an instance of Reset and thus will be
inferred in FIRRTL.

Rest of the binding refactor