tiergraph 0.3.0

Added

• Added JSON-valued attributes. JsonType.JSON declares an attribute whose
  value is ordinary JSON -- null, a boolean, an integer, a double, a string,
  an array or an object -- held as JsonAttributeValue on any carrier a
  scalar attribute can sit on. The value is owned: the attribute keeps its own
  snapshot, so a caller mutating the container it passed, or the one it is
  handed back, changes neither. Kinds and float signs are part of the value's
  identity, so 0, 0.0 and False are three values and -0.0 crosses the
  wire as -0.0.
• BREAKING: FORMAT_VERSION moves from "0.2.0" to "0.3.0". This
  reader accepts only its own format version, so it refuses a document written
  at "0.2.0" with a discriminator refusal. The release line moves because
  the schema grows by the JSON attribute record; otherwise its shape is
  unchanged against the 0.2 line.

Changed

• A JSON integer must lie within +/-(2**53 - 1), the range a JSON peer decodes
  exactly. JSON has one number type and its peers read it as an IEEE double,
  so a larger integer reads back as a different number with no error.
  JsonAttributeValue refuses one where it is built, and reading a document
  refuses one as a staged Refusal rather than loading a value its peers
  would corrupt.
• Reading a document refuses an integer literal too long to convert as a
  staged Refusal, where it previously raised a bare ValueError from the
  process's digit limit. This bounds conversion only; it places no budget on
  integer fields other than JSON attribute values.
• The scalar attribute constructor and a grammar weight refuse a JSON-typed
  declaration, so a JSON value cannot reach a place that reads only scalars.