XPath Handling

For https://github.com/YorickPeterse/oga/issues/9 Oga needs to be able to parse XPath queries into an AST. Said AST then has to be consumed in order to figure out what nodes to pull from a DOM document.

For this the following would have to be added:

• Lexer
  • Lexing of all known XPath tokens
  • Tests for the primitive types (strings, integers and floats)
  • Tests for every known axis
  • Benchmarks
  • Lex wildcards as identifiers, not T_STAR
• Parser
  • Distinguish absolute and relative paths from each other on AST level
  • Parsing of function calls (inc arguments)
  • Parsing of all the available operators
  • Add token precedence for all the operator tokens (T_EQ, T_OR, etc)
  • Ensure behaviour of when certain nodes are allowed (or not) resembles the rules implied by libxml/Nokogiri (this assumes libxml is correct in this)
  • Tests for every known axis
  • Benchmarks
• AST consumer
  • Axis evaluation
    • ancestor
    • ancestor-or-self
    • attribute
    • child
    • descendant
    • descendant-or-self
    • following
    • following-sibling
    • namespace
    • parent
    • preceding
    • preceding-sibling
    • self
  • Functions
    • last
    • position
    • count
    • id
    • local-name
    • namespace-uri
    • name
    • string
    • concat
    • starts-with
    • contains
    • substring-before
    • substring-after
    • substring
    • string-length
    • normalize-space
    • translate
    • boolean
    • not
    • true
    • false
    • lang
    • number
    • sum
    • floor
    • ceiling
    • round
  • Node type tests
    • node
    • text
    • comment
    • processing-instruction
  • Operators
    • |
    • and
    • or
    • +
    • div
    • mod
    • =
    • !=
    • <
    • >
    • <=
    • >=
    • *
    • -
  • Variable bindings
  • Benchmarks
  • Tests

The AST consumer would be fed a DOM document and, based on the XPath query, returns a list of nodes.