implement: unit identity -- dotted paths, name rules, source path

Units start here, with the part that needs no import graph: what a unit is
called and where it must live.

The parser only ever read a single identifier after `unit` and `import`, so
`unit game.main;` and `import std.io;` were syntax errors -- which is why the
dotted fixtures failed at the semicolon. It now reads a dotted path and stores
it canonically, dots included, since that spelling is the unit's identity
everywhere else. `import a.b as c;` parses too, with the alias on the node.

resolve.c is the new pass between parsing and checking, for the questions that
span files. It carries SPEC 8.1 so far: each path segment is ASCII lowercase,
starts with a letter, continues with letters, digits or underscore, and is at
most eight characters; and the dotted path must match the source path it was
read from, so game.world.map has to come from game/world/map.fe. The source
side is folded to lowercase before comparing, because a case-insensitive host
must not let two spellings become two units.

That rule then applied to the fixtures, which were not obeying it: 57 declared
a unit name unrelated to their file, left over from the milestone directories,
and eight had names too long to be legal. Both are now aligned -- the rule is
worth having only if the tree follows it.

units: badupper, badlong and unitbad pass. 138 -> 146 of 188. The rest of
units/ needs the import graph, which is the next piece: resolution, cycles,
bindings and visibility.

fec/tests/format/bad_ari.fe

@@ -1,4 +1,4 @@
-unit m4_bad_arity;
+unit bad_ari;
 
 fn main() -> i32 {
     @print("{} {}", 1);

fec/tests/format/bad_bufw.fe

@@ -1,4 +1,4 @@
-unit m4_bad_buffer_writer;
+unit bad_bufw;
 
 fn main() -> void {
     var raw: [4]u8 = [0, 0, 0, 0];

fec/tests/format/bad_cls.fe

@@ -1,4 +1,4 @@
-unit m4_bad_cls;
+unit bad_cls;
 
 fn main() -> i32 {
     @print("}", 1);

fec/tests/format/bad_many.fe

@@ -1,4 +1,4 @@
-unit m4_bad_many;
+unit bad_many;
 
 fn main() -> i32 {
     @print("{}", 1, 2);

fec/tests/format/bad_open.fe

@@ -1,4 +1,4 @@
-unit m4_bad_open;
+unit bad_open;
 
 fn main() -> i32 {
     @print("{", 1);

fec/tests/format/bad_run.fe

@@ -1,4 +1,4 @@
-unit m4_bad_runtime;
+unit bad_run;
 
 fn main() -> i32 {
     var fmt: str = "{}";

fec/tests/format/bad_try.fe

@@ -1,4 +1,4 @@
-unit m4_bad_try;
+unit bad_try;
 
 fn main() -> i32 {
     try @print("nope");

fec/tests/format/bad_type.fe

@@ -1,4 +1,4 @@
-unit m4_bad_type;
+unit bad_type;
 
 struct Point { x: i32, }
 

fec/tests/format/bad_verb.fe

@@ -1,4 +1,4 @@
-unit m4_bad_verb;
+unit bad_verb;
 
 fn main() -> i32 {
     @print("{q}", 1);

fec/tests/format/bad_writ.fe

@@ -1,4 +1,4 @@
-unit m4_bad_writer;
+unit bad_writ;
 
 fn main() -> i32 {
     var x: i32 = 0;

fec/tests/format/ok_forma.fe

@@ -1,4 +1,4 @@
-unit m4_format;
+unit ok_forma;
 
 const FMT: str = "n={} hex={x} c={c} s={s} b={b} {{ok}}\n";
 

fec/tests/format/ok_prop.fe

@@ -1,4 +1,4 @@
-unit m4_prop;
+unit ok_prop;
 
 pub fn propagate(w: io.Writer) -> !void {
     try @fprint(w, "a{}b", 1);

fec/tests/format/ok_try_f.fe

@@ -1,4 +1,4 @@
-unit m4_try_fprint;
+unit ok_try_f;
 
 fn main() -> !void {
     var raw: [4]u8 = [0, 0, 0, 0];