Further module system revamp (fix #659)

To import a module now use:

    # Import module.jq file:
    import "relative/path/to/module" as foo;

    # Use the module's defs as foo::<def-name>

To import a JSON file:

    # Read file.json:
    import "relative/path/to/file" as $foo;
    #
    # Use as $foo::foo

Using `-L` now drops the builtin library path and appends the requested
path to the empty array (or the result of an earlier `-L`).

Support for the `$JQ_LIBRARY_PATH` environment variable has been
removed.

compile.h

@@ -20,6 +20,7 @@ block gen_noop();
 int block_is_noop(block b);
 block gen_op_simple(opcode op);
 block gen_const(jv constant);
+block gen_const_global(jv constant, const char *name);
 int block_is_const(block b);
 jv_kind block_const_kind(block b);
 jv block_const(block b);
@@ -28,9 +29,9 @@ block gen_op_unbound(opcode op, const char* name);
 block gen_op_bound(opcode op, block binder);
 block gen_op_var_fresh(opcode op, const char* name);
 
-block gen_module(const char* name, block metadata);
+block gen_module(block metadata);
 jv block_module_meta(block b);
-block gen_import(const char* name, block metadata, const char *as);
+block gen_import(const char* name, block metadata, const char *as, int is_data);
 block gen_function(const char* name, block formals, block body);
 block gen_param_regular(const char* name);
 block gen_param(const char* name);