perf(buffer): unify Buffer::write_bytes with write_bytesview

`Buffer::write_bytes(value : Bytes)` and `Buffer::write_bytesview(value : BytesView)`
were doing the same thing with different parameter types. Expand the
former to `BytesView` and make it delegate — existing `Bytes` callers
keep working via coercion, and the implementation is now one line
instead of two parallel blit loops kept in sync.

`write_bytesview` stays for now (callers may have reached for the
explicit name); in a future cleanup it can be deprecated once downstream
migrates to just using `write_bytes`.

Continues the view-safe expansion theme (#3459, #3460, #3461).

Author: bobzhang

buffer/buffer.mbt

@@ -133,12 +133,12 @@ pub fn new(size_hint? : Int = 0) -> Buffer {
 
 ///|
 /// Create a buffer from a bytes.
-pub fn from_bytes(bytes : Bytes) -> Buffer {
+pub fn from_bytes(bytes : BytesView) -> Buffer {
   let val_len = bytes.length()
   let buf = new(size_hint=val_len)
   // inline write_bytes, skip grow_if_necessary check
   // SAFETY: known bytes size
-  buf.data.blit_from_bytes(0, bytes, 0, val_len)
+  buf.data.blit_from_bytes(0, bytes.data(), bytes.start_offset(), val_len)
   buf.len = val_len
   buf
 }
@@ -737,11 +737,8 @@ pub fn Buffer::write_object(self : Buffer, value : &Show) -> Unit {
 ///   )
 /// }
 /// ```
-pub fn Buffer::write_bytes(self : Buffer, value : Bytes) -> Unit {
-  let val_len = value.length()
-  self.grow_if_necessary(self.len + val_len)
-  self.data.blit_from_bytes(self.len, value, 0, val_len)
-  self.len += val_len
+pub fn Buffer::write_bytes(self : Buffer, value : BytesView) -> Unit {
+  self.write_bytesview(value)
 }
 
 ///|

buffer/pkg.generated.mbti

@@ -8,7 +8,7 @@ import {
 // Values
 pub fn from_array(ArrayView[Byte]) -> Buffer
 
-pub fn from_bytes(Bytes) -> Buffer
+pub fn from_bytes(BytesView) -> Buffer
 
 pub fn from_iter(Iter[Byte]) -> Buffer
 
@@ -28,7 +28,7 @@ pub fn Buffer::to_bytes(Self) -> Bytes
 pub fn Buffer::to_repr(Self) -> @debug.Repr
 pub fn Buffer::view(Self) -> ArrayView[Byte]
 pub fn Buffer::write_byte(Self, Byte) -> Unit
-pub fn Buffer::write_bytes(Self, Bytes) -> Unit
+pub fn Buffer::write_bytes(Self, BytesView) -> Unit
 pub fn Buffer::write_bytesview(Self, BytesView) -> Unit
 pub fn Buffer::write_char_utf16be(Self, Char) -> Unit
 pub fn Buffer::write_char_utf16le(Self, Char) -> Unit