o Basic builder of a struct.

o Owned builder.
o Immutable builder.

o skip setters
o skip all setters; enable particular setters.
o custom setters.
o custom all setters; enable particular setters.

o Override setter visibility
o Override builder visibility

o Rename builder.

o Setter prefix
o Setter renaming

o Into setters (field or struct)

o Strip_option setters

o Fallible setters

o Default values

o rename or skip build function

o pre-build validation

o Replace Error type

o Override field type.

o Override field builder.

o Perhaps, tolerate the possibility that Option, Result, String, etc might not be in scope.

- Set more attributes on builder.

o Copy attributes from fields.

o Pass-through attributes

o Documentation (generated)

o Documentation (internal)

o Rename setter(default)

o Subbuilders

==============================

- Support tuple and unit structs.


==============================

Derive deftly:
  o Simpler way to make "default" settings for tmeta. #40 
  o Document that ${define A {B C D}} needs those inner braces.
  - Fresh unused/hygienic identifier??
  - Builtin stringify (not just ${ as str})
  - derive_deftly(A::Foo) should work, and apparently does.  Document this?
  - comments: inner vs outer?
  o "User-derived expansion/condition not found?" SAY WHICH. #41
  - Documentation templating.

  o Look for a better way to avoid this:
     ${if foo {${define BAR { ... } }} else { ${define BAR {...}}}}
    #42