[gnidan]

Implements #291.

This adds a region with "location": "virtual" for values that no EVM data location stores, e.g. a variable the optimizer removed. The pointer schema's dispatch does not change: a virtual region is just another region, and dereferencing still yields regions only.

Every region is base fields plus one addressing scheme. Virtual regions use a new third scheme, ethdebug/format/pointer/scheme/fragment: a width-bearing expression supplies the byte space (the expression's value), and optional offset (default 0) and length (default: to the end of the value) address into it. A fragment outside the value's bytes is invalid. The expression is evaluated against the same machine state as the pointer's other regions and may refer only to regions before it. Lookups follow the region's fields, so .offset and .length work and .slot does not; ~read gives the fragment's bytes.

group:
  - name: a
    location: stack
    slot: 2
  - name: x
    location: virtual
    expression:
      ~wordsized:
        ~sum:
          - ~read: a
          - 1

The reference implementation evaluates the expression while it makes the region concrete, keeps the bytes on the concrete region, and fills in the default offset and length; read returns the fragment. A new integration case reads a + 1 and the first 4 bytes of keccak256(a) from a storage variable over a solc trace.

Stacked on #408.

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[gnidan]

@djolertrk @cameel would be curious what you think of this approach to solving #291