memcopy! and memset! intrinsics

LANGUAGE.md

@@ -180,6 +180,25 @@ infinity/NaN behavior.
 Only these six division bang calls select integer-division behavior. Bare calls such as
 `divfloor(a, b)` and qualified calls such as `math.divfloor(a, b)` resolve to ordinary functions.
 
+#### typed memory operations
+
+`memcopy!(destination, source)` and `memset!(destination, value)` are available at runtime and
+comptime. A destination must be a mutable slice or mutable pointer-to-array. A `memcopy!` source
+may be a slice or pointer-to-array; many-item pointers must first be sliced. Array pointers are
+treated as regions containing their explicit logical elements, including a sentinel only when it
+is part of that array region.
+
+`memcopy!` requires the same element type after alias resolution and the same element count. Its
+non-empty regions must not overlap. Comptime calls diagnose unequal lengths and overlap; runtime
+calls trap for either condition or if the element count cannot be converted to a byte count.
+Zero-sized elements still require equal counts. Empty copies are no-ops and may name the same
+region.
+
+`memset!` coerces `value` to the destination element type. Use `memset!(destination, 0)` to zero a
+region; there is no separate `memzero!`, and `memset!` does not promise secure zeroing. Copying or
+filling with `undefined` transfers undefined state without reading it. Each operand is evaluated
+exactly once. Bare functions named `memcopy` or `memset` remain ordinary user functions.
+
 ### functions, C interop, and linking
 
 - demand-monomorphized Brolang and C-ABI functions