Implement #use directive for dependency management

Introduces the `#use` preprocessor directive, allowing Myc scripts to
explicitly declare dependencies on other Myc libraries. This change
moves dependency management from a command-line argument to an in-script
declaration, ensuring scripts are self-contained and can correctly
resolve macros and other symbols.

Key features include:
- Declarations at the beginning of a file, evaluated before parsing.
- Support for relative paths and a new `->` separator.
- Automatic resolution of dependencies from specified search paths.
- Idempotent loading to prevent duplicate parsing and evaluation.
- No AST pollution; dependency management is a compile-time concern.

The compiler's lexer has been updated to recognize `#` as a comment
character, and the environment now manages search paths and loaded
modules. This lays the groundwork for more complex library structures
and improved code organization.

examples/sma.myc

@@ -1,4 +1,5 @@
-;; Output: 5.5
+#use lib->sma
+;; Output: void
 
 (do
   (def sma20 (SMA 20))