Document single-arg lambda design in ADR

adrs/003_single_arg_lambdas_in_ast.md

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+# ADR-003: Use single-argument lambdas in the AST
+
+**Date**: 26/09/2023  
+**Status**: Current
+
+## Decision
+
+Always treat lambdas/functions in the abstract syntax tree as if they only take
+a single argument.
+
+## Context
+
+While spiking an implementation of a bidirectional typechecking algorithm, it
+became clear that having lambdas with multiple arguments represented in the AST
+was making things unnecessarily complicated, by requiring the type checking and
+inference functions to take multiple "steps" at once.
+
+
+## Tradeoffs
+
+Since we can easily build single argument lambdas from the current surface 
+syntax, and since it's relatively trivial to walk the AST to "gather up" nested
+lambdas into a single multi-arg closure in the bytecode, there's no real
+tradeoff here. It simplifies the AST at no major cost.