zstd_opt: simplified ZSTD_getPrice() and ZSTD_updatePrice() interface

ZSTD_getPrice() and ZSTD_updatePrice() accept normal matchlength as argument
instead of matchlength-MINMATCH,
which makes them easier / more logical to use and read.
Conversion is simply done internally.

lib/compress/zstd_compress_internal.h

@@ -91,7 +91,7 @@ typedef struct {
     U32  log2offCodeSum;         /* pow2 to compare log2(offreq) to */
     U32  factor;                 /* added to calculate ZSTD_getPrice() (but why?) */
     /* end : updated by ZSTD_setLog2Prices */
-    U32  staticPrices;           /* ? do not update statistics ? */
+    U32  staticPrices;           /* prices follow a static cost structure, statistics are irrelevant */
     U32  cachedPrice;
     U32  cachedLitLength;
     const BYTE* cachedLiterals;