Add bidirectional bulk SPI transfer, update SdFAT (#819)

Should speed up SD transfers significantly (2.5x+).

See #801

cores/rp2040/api/HardwareSPI.h

@@ -110,6 +110,19 @@ class HardwareSPI
     virtual uint16_t transfer16(uint16_t data) = 0;
     virtual void transfer(void *buf, size_t count) = 0;
 
+    // New transfer API.  If either send or recv == nullptr then ignore it
+    virtual void transfer(const void *send, void *recv, size_t count) {
+        const uint8_t *out = (const uint8_t *)send;
+        uint8_t *in = (uint8_t *)recv;
+        for (size_t i = 0; i < count; i++) {
+            uint8_t t = out ? *(out++) : 0xff;
+            t = transfer(t);
+            if (in) {
+                *(in++) = t;
+            }
+        }
+    }
+
     // Transaction Functions
     virtual void usingInterrupt(int interruptNumber) = 0;
     virtual void notUsingInterrupt(int interruptNumber) = 0;

ESP8266SdFat @ 255180be

-Subproject commit 43a65b42d5a827092188cfe11fb27237da252692
+Subproject commit 255180be140d252605f28667078201fba588b968

libraries/SPI/src/SPI.cpp

@@ -141,13 +141,13 @@ void SPIClassRP2040::transfer(void *buf, size_t count) {
     DEBUGSPI("SPI::transfer completed\n");
 }
 
-void SPIClassRP2040::transfer(void *txbuf, void *rxbuf, size_t count) {
+void SPIClassRP2040::transfer(const void *txbuf, void *rxbuf, size_t count) {
     if (!_initted) {
         return;
     }
 
     DEBUGSPI("SPI::transfer(%p, %p, %d)\n", txbuf, rxbuf, count);
-    uint8_t *txbuff = reinterpret_cast<uint8_t *>(txbuf);
+    const uint8_t *txbuff = reinterpret_cast<const uint8_t *>(txbuf);
     uint8_t *rxbuff = reinterpret_cast<uint8_t *>(rxbuf);
 
     // MSB version is easy!

libraries/SPI/src/SPI.h

@@ -36,7 +36,7 @@ public:
     void transfer(void *buf, size_t count) override;
 
     // Sends one buffer and receives into another, much faster! can set rx or txbuf to NULL
-    void transfer(void *txbuf, void *rxbuf, size_t count);
+    void transfer(const void *txbuf, void *rxbuf, size_t count) override;
 
     // Call before/after every complete transaction
     void beginTransaction(SPISettings settings) override;