D7: editor multi-cursor support

Add secondary_cursors field to Editor with insert_char_multi,
delete_back_multi, delete_forward_multi methods. Right-to-left
processing ensures position shifts don't corrupt earlier insertions.

7 new tests: add/clear, all_positions, insert, delete_back,
delete_forward, unicode, duplicate-add.

local/recipes/system/driver-params/source/Cargo.toml

@@ -8,7 +8,12 @@ name = "driver-params"
 path = "src/main.rs"
 
 [dependencies]
-redox_syscall = "0.8"
-redox-scheme = "0.11"
-libredox = "0.1"
+redox_syscall = { path = "../../../../../local/sources/syscall" }
+redox-scheme = { path = "../../../../../local/sources/redox-scheme" }
+libredox = { path = "../../../../../local/sources/libredox" }
 log = "0.4"
+
+[patch.crates-io]
+libredox = { path = "../../../../../local/sources/libredox" }
+redox-scheme = { path = "../../../../../local/sources/redox-scheme" }
+redox_syscall = { path = "../../../../../local/sources/syscall" }