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.

opcode_list.h

@@ -6,6 +6,7 @@ OP(POP,   NONE,     1, 0)
 OP(LOADV, VARIABLE, 1, 1)
 OP(LOADVN, VARIABLE, 1, 1)
 OP(STOREV, VARIABLE, 1, 0)
+OP(STORE_GLOBAL, GLOBAL, 0, 0)
 OP(INDEX, NONE,     2, 1)
 OP(INDEX_OPT, NONE,     2, 1)
 OP(EACH,  NONE,     1, 1)