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fixed-point: playing it safe. SHL32() now automatically casts input to 32-bit
to prevent surprises. Also, a few comments.

--- a/libcelt/fixed_generic.h

+++ b/libcelt/fixed_generic.h

@@ -54,7 +54,7 @@

 #define SHR16(a,shift) ((a) >> (shift))

 #define SHL16(a,shift) ((a) << (shift))

 #define SHR32(a,shift) ((a) >> (shift))

-#define SHL32(a,shift) ((a) << (shift))

+#define SHL32(a,shift) ((celt_word32_t)(a) << (shift))

 #define PSHR16(a,shift) (SHR16((a)+((1<<((shift))>>1)),shift))

 #define PSHR32(a,shift) (SHR32((a)+((EXTEND32(1)<<((shift))>>1)),shift))

 #define VSHR32(a, shift) (((shift)>0) ? SHR32(a, shift) : SHL32(a, -(shift)))

--- a/libcelt/mathops.h

+++ b/libcelt/mathops.h

@@ -54,16 +54,13 @@

 #include "entcode.h"

+/** Integer log in base2. Undefined for zero and negative numbers */

 static inline celt_int16_t celt_ilog2(celt_word32_t x)

 {

    return EC_ILOG(x)-1;

 }

-#define C0 3634

-#define C1 21173

-#define C2 -12627

-#define C3 4204

-

+/** Sqrt approximation (QX input, QX/2 output) */

 static inline celt_word32_t celt_sqrt(celt_word32_t x)

 {

    int k;

@@ -149,7 +146,7 @@

 #define D1 11356

 #define D2 3726

 #define D3 1301

-/* Input in Q11 format, output in Q16 */

+/** Base-2 exponential approximation (2^x). (Q11 input, Q16 output) */

 static inline celt_word32_t celt_exp2(celt_word16_t x)

 {

    int integer;

@@ -164,6 +161,7 @@

    return VSHR32(EXTEND32(frac), -integer-2);

 }

+/** Reciprocal approximation (Q15 input, Q16 output) */

 static inline celt_word32_t celt_rcp(celt_word32_t x)

 {

    int i, neg=0;