405 - permit loading just low levels of codebase

When I'm doing extensive surgery to the internals I want to avoid
loading higher levels; they aren't expected to work. But I don't want to
keep different levels in separate files just for that. And I definitely
don't want to put low-level stuff first. Now I can influence loading in
a cross-cutting manner by creating sections with numbers:

  (section 100
    ...code...)

And disabling them by running:

  $ ./anarki/arc 99 mu.arc.t

Currently we load all mu 'system software' in level 100, so running at
level 99 sidesteps them. Lower levels coming soon.

But most of the time we don't need to worry about levels, and the 'mu'
script lets us forget about them. Just run .mu files with:

  $ ./mu factorial.mu

To run tests:

  $ ./mu test mu.arc.t

1 files changed, 4 insertions, 4 deletions

diff --git a/Readme.md b/Readme.md
index 29b1a224..4b7a4a49 100644
--- a/Readme.md
+++ b/Readme.md
@@ -113,7 +113,7 @@ the system doesn't recognize gets silently ignored.
 Try this program out now:
 
 ```shell
-  $ ./anarki/arc mu.arc factorial.mu
+  $ ./mu factorial.mu
   result: 120  # factorial of 5
   ...  # ignore the memory dump for now
 ```
@@ -159,7 +159,7 @@ inserting code at them.
 Another example, this time with concurrency.
 
 ```shell
-  $ ./anarki/arc mu.arc fork.mu
+  $ ./mu fork.mu
 ```
 
 Notice that it repeatedly prints either '34' or '35' at random. Hit ctrl-c to
@@ -168,7 +168,7 @@ stop.
 Yet another example forks two 'routines' that communicate over a channel:
 
 ```shell
-  $ ./anarki/arc mu.arc channel.mu
+  $ ./mu channel.mu
   produce: 0
   produce: 1
   produce: 2
@@ -200,7 +200,7 @@ allocator and a few other places).
 Try running the tests:
 
 ```shell
-  $ ./anark/arc mu.arc.t
+  $ ./mu test mu.arc.t
   $  # all tests passed!
 ```