boot: methods (fn T.m, self by address, mutation analysis), str() of any value

Methods are functions whose first parameter is the receiver's address; a method
that changes self can't be called on a temporary or a read-only value, and
calling one on a variable keeps a parameter from being borrowed. An argument
that lives inside the receiver is copied (c.merge(c)). str(x) and print of
arrays and structs go through one formatter, as in the C++ compiler.

boot.sh: 12 passed, 0 failed, 10 skipped.

boot/ast.fch

@@ -75,6 +75,7 @@ fn show(Type t) -> str {
 struct Node {
     str kind
     str name      // identifier, operator, callee, field or method name
+    str recv      // fn Point.move(...): "Point"; empty for a plain function
     str text      // literal text
     Type ty       // declared type (decl, param, field, fn result)
     bool flag     // decl: has a type; fn: has a body