Fix Print and File incompatible writes w/casts

Print and File have ambiguous resolutions for single-argument
write(0)s.

Fix by adding explicit methods.  A write of any integer will not be a
const char* write (i.e. won't write a string) but will instead just
write the integer truncated to 8 bits (as makes sense).

cores/esp8266/FS.h

@@ -54,6 +54,11 @@ class File : public Stream
     // Print methods:
     size_t write(uint8_t) override;
     size_t write(const uint8_t *buf, size_t size) override;
+    size_t write(int8_t t) { return write((uint8_t)t); }
+    size_t write(int16_t t) { return write((uint8_t)(t & 0xff)); }
+    size_t write(int32_t t) { return write((uint8_t)(t & 0xff)); }
+    size_t write(uint16_t t) { return write((uint8_t)(t & 0xff)); }
+    size_t write(uint32_t t) { return write((uint8_t)(t & 0xff)); }
 
     // Stream methods:
     int available() override;

tests/host/fs/test_fs.cpp

@@ -350,4 +350,12 @@ TEST_CASE("Multisplendored File::writes", "[fs]")
     f.write((const uint8_t*)&bigone, 4);
     f.close();
     REQUIRE(readFileSD("/file.txt") == "aAbbcctheendxyz@@@@");
+    File g = SD.open("/file.txt", FILE_WRITE);
+    g.write(0);
+    g.close();
+    g = SD.open("/file.txt", FILE_READ);
+    uint8_t u = 0x66;
+    g.read(&u, 1);
+    g.close();
+    REQUIRE(u == 0);
 }