Add --dont-fold option to disable folding specific prim ops

[seldridge]

This adds a --dont-fold options (backed by a DisableFold annotation)
that lets a user specify primitive operations which should never be
folded. This feature lets a user disable certain folds which may be
allowable in FIRRTL (or by any sane synthesis tool), but due to inane
Verilog language design causes formal equivalence tools to fail due to
the fold.

Add a test that a user can disable a / a -> 1 with a
DisableFold(PrimOps.Div) annotation.

Fixes #2029.

Example Usage

The following circuit:

circuit Foo :
  module Foo :
    input a: UInt<8>
    output b: UInt<8>

    b <= div(a, a)

When normally compiled will have div(a, a) legally optimized to 1 by taking advantage of the fact that division by zero is undefined in the FIRRTL spec (and arguably is implicitly undefined for synthesis in Verilog...):

firrtl -i Foo.fir

module Foo(
  input  [7:0] a,
  output [7:0] b
);
  assign b = 8'h1;
endmodule

If a user disables folding of division:

firrtl -i Foo.fir --dont-fold div

The user gets an a / a in the output:

module Foo(
  input  [7:0] a,
  output [7:0] b
);
  assign b = a / a;
endmodule

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• [n/a] Did you update the FIRRTL spec to include every new feature/behavior?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you state the API impact?
• Did you specify the code generation impact?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• new feature/API

API Impact

This is a pure command line / annotation API addition.

Backend Code Generation Impact

None by default. A user can elect to change the backend code generation with the new --dont-fold option.

Desired Merge Strategy

• Squash: The PR will be squashed and merged (choose this if you have no preference.

Release Notes

• Add the --dont-fold option to disable folding a specific primop

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels?
• Did you mark the proper milestone (1.2.x, 1.3.0, 1.4.0) ?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you mark as Please Merge?

[jackkoenig]

Some pretty big nits, but otherwise this looks good!