Tracking issue: document bitvector rules

This tracks the documentation of the bitvector rules to the specification document.

Rough TODOs:

• Collect all note documents about bitvector rules
• Add bitvector section to "concepts" section of the specification
• Add all bitvector rules
• Add examples

Bitvector rules:

• bitblast_var
• bitblast_const
• bitblast_and
• bitblast_or
• bitblast_xor
• bitblast_xnor
• bitblast_not
• bitblast_add
• bitblast_neg
• bitblast_mult
• bitblast_extract
• bitblast_concat
• bitblast_eq
• bitblast_ult
• bitblast_slt
• bitblast_sext