New floating-point conversion routines

Substitute in nasm64developer's "acfloat4" routine.  This
floating-point conversion routine is not perfect (it gets a fair
number of LSB errors), but the old NASM code was just plain broken.
nasm64developer's code at least gets within ±1 LSB.

float.h

@@ -10,7 +10,14 @@
 #ifndef NASM_FLOAT_H
 #define NASM_FLOAT_H
 
-int float_const(char *number, int32_t sign, uint8_t *result, int bytes,
+enum float_round {
+    FLOAT_RC_NEAR,
+    FLOAT_RC_ZERO,
+    FLOAT_RC_DOWN,
+    FLOAT_RC_UP,
+};
+
+int float_const(const char *string, int sign, uint8_t *result, int bytes,
                 efunc error);
 
 #endif