Support minimal incremental interface

[DavePearce]

A minimal incremental interface has the following qualities:

• Treats a WhileyFile instance as a sequence of bytes
• Supports modification through Deltas.
• Propagates Deltas through the internal structures.
• Can recompute all internal structures from scratch if necessary.

That means we need to incrementalise the internal structures. In essence, we have a few:

1. (Lexer). This maps u8 sequences to Token sequences.
2. (Parser). This maps Token sequences to SyntacticHeap instances.
3. (Type Checker). This infers supplementary information from the SyntacticHeap.

The starting point is perhaps to test out a simple mapping from byte sequences to tokens using delta_inc.