Remove bad UDT implementation.
Resolve imports sanelier.

The definitions in scope in a program are now all those transitively
reachable via 'import' directives anywhere in the set of loaded modules.
This is still a bit excessive due to the lack of namespacing, but it
lays groundwork for further improvements.
Merge different AST types and kill 'AST' class.

There is now a single 'Tree' type for parsed, resolved, and typed
ASTs. It is parameterized by the types of labels ('Text', 'Name') and
metadata ('Location', 'Type').
Remove unused 'Named' constructor of 'Type'.
Require type annotations on definitions.
WIP Upgrade surface syntax.

Done:

 - Parse parenthesized expressions as infix.

 - Allow fixity declarations for user-defined operators.

 - Assume user-defined operators are infix, left-associative, and have
   precedence 6.

 - Update examples and tests to use infix generally.

 - Parse a parenthesized operator as postfix.

 - Start making prelude non-special.

To do:

 - Make infix the default.

 - Enforce that precedence is from 1 to 9 or.

 - Enforce no duplicate/conflicting operator declarations.

 - Give all prefix and postfix operators the same precedence.

 - Add semicolon pseudo-operator.

 - Change "-> { x y ... }" syntax to "-> x y ...;".

 - Kill the backslash syntax and parse mixfix functions.
Finish making prelude non-special.
WIP More syntactic improvements.

Relevant:

 - Make infix default.

 - Add semicolon pseudo-operator.

 - Change local syntax from "->{ x y ... }" to "-> x y ...;".

 - Add section syntax:

       (_ - 5)    5 (-)
       (5 - _)    5 swap (-)
       (_ - _)    (-)

 - Refactor the prelude accordingly.

Earmarked:

 - Remove unit type and syntax.

 - Remove unused 'Prelude_Unit' and 'Prelude_Effect' modules.

Yet to do:

 - Enforce invariants.

 - Improve error messages.

 - Allow "_" placeholder as local name to drop a value.
Rename 'row' to 'stack'.
Rename 'function' to 'quotation'.
Remove '@' operator and improve vector manipulation operators.
Enforce stack effects and remove '(unknown)' source location.
Improve stack effect hint checking.
Allow placeholder syntax to ignore a local.
Remove '--no-implicit-prelude' on examples.
Operator wibbles.
Tuple wibbles.
Enforce precedence. Non-infix implies highest precedence.
Remove word case distinction and explicitly quantify types.
Remove non-infix operators to improve section syntax.