refactor: Mark some static global vars as const

These were found while looking for static mutable state in the kernel
library.

Author: TheCharlatan

src/core_read.cpp

@@ -245,7 +245,7 @@ bool ParseHashStr(const std::string& strHex, uint256& result)
 
 util::Result<int> SighashFromStr(const std::string& sighash)
 {
-    static std::map<std::string, int> map_sighash_values = {
+    static const std::map<std::string, int> map_sighash_values = {
         {std::string("DEFAULT"), int(SIGHASH_DEFAULT)},
         {std::string("ALL"), int(SIGHASH_ALL)},
         {std::string("ALL|ANYONECANPAY"), int(SIGHASH_ALL|SIGHASH_ANYONECANPAY)},

src/script/descriptor.cpp

@@ -116,13 +116,13 @@ std::string DescriptorChecksum(const Span<const char>& span)
      * As a result, within-group-of-32 errors count as 1 symbol, as do cross-group errors that don't affect
      * the position within the groups.
      */
-    static std::string INPUT_CHARSET =
+    static const std::string INPUT_CHARSET =
         "0123456789()[],'/*abcdefgh@:$%{}"
         "IJKLMNOPQRSTUVWXYZ&+-.;<=>?!^_|~"
         "ijklmnopqrstuvwxyzABCDEFGH`#\"\\ ";
 
     /** The character set for the checksum itself (same as bech32). */
-    static std::string CHECKSUM_CHARSET = "qpzry9x8gf2tvdw0s3jn54khce6mua7l";
+    static const std::string CHECKSUM_CHARSET = "qpzry9x8gf2tvdw0s3jn54khce6mua7l";
 
     uint64_t c = 1;
     int cls = 0;