feat: infer complete ptrcast target types

testbed/mem_alloc/std/mem/mem.bro

@@ -30,7 +30,7 @@ raw_free func(allocator Allocator, memory ?*mut u8, size usize, alignment usize)
 @hide empty_storage [1]mut u64 := [0]
 
 @hide empty_slice func($T type, count usize) []mut T {
-	pointer *mut T :: ptrcast!(T, (&empty_storage).ptr)
+	pointer *mut T :: ptrcast!((&empty_storage).ptr)
 	return pointer[..count]
 }
 
@@ -49,7 +49,7 @@ alloc func($T type, allocator Allocator, count usize) []mut T ! AllocError {
 
 	memory ?*mut u8 := raw_alloc(allocator, count * element_size, alignof!(T))
 	if memory |bytes| {
-		pointer *mut T :: ptrcast!(T, bytes)
+		pointer *mut T :: ptrcast!(bytes)
 		return pointer[..count]
 	}
 	return .out_of_memory
@@ -57,7 +57,7 @@ alloc func($T type, allocator Allocator, count usize) []mut T ! AllocError {
 
 free func($T type, allocator Allocator, memory []mut T) void {
 	if memory.len != 0 and sizeof!(T) != 0 {
-		raw_free(allocator, ptrcast!(u8, memory.ptr), memory.len * sizeof!(T), alignof!(T))
+		raw_free(allocator, ptrcast!(memory.ptr), memory.len * sizeof!(T), alignof!(T))
 	}
 }
 
@@ -86,7 +86,7 @@ free func($T type, allocator Allocator, memory []mut T) void {
 	}
 
 	if alignment <= malloc_alignment {
-		return ptrcast!(u8, c.malloc(c_ulong(size)))
+		return ptrcast!(c.malloc(c_ulong(size)))
 	}
 
 	memory [1]mut ?*mut anyopaque := [null]
@@ -95,7 +95,7 @@ free func($T type, allocator Allocator, memory []mut T) void {
 		return null
 	}
 
-	return ptrcast!(u8, memory[0])
+	return ptrcast!(memory[0])
 }
 
 @hide c_realloc func(_ ?@mut anyopaque, memory ?*mut u8, old_size usize, new_size usize, alignment usize) ?*mut u8 {
@@ -110,7 +110,7 @@ free func($T type, allocator Allocator, memory []mut T) void {
 
 	if memory |old_memory| {
 		if alignment <= malloc_alignment {
-			return ptrcast!(u8, c.realloc(old_memory, c_ulong(new_size)))
+			return ptrcast!(c.realloc(old_memory, c_ulong(new_size)))
 		}
 
 		new_memory ?*mut u8 := c_alloc(null, new_size, alignment)