Speed up Slice::ToString(bool hex=true)

[thatsafunnyname]

This issue created for #15045

• Replace branching with a single indexed load from constexpr lookup table.
• Instead of reserve + push_back, one allocation sets final size immediately, avoiding repeated size updates and bounds checks.
• Raw pointer skips repeated operator[] overhead and internal index tracking.
• When hex=false, construct the string directly, avoiding assign + return copy.

Noticed when using ldb --value_hex scan:

Before:

perf stat -e branches,branch-misses tools/ldb --value_hex scan --db=test_db | dd of=/dev/null
 Performance counter stats for 'tools/ldb --value_hex scan --db=test_db':
     2,983,373,019      branches:u
       101,288,380      branch-misses:u           #    3.40% of all branches
       4.365280485 seconds time elapsed
       2.854659000 seconds user
       0.741348000 seconds sys
884520+1 records in
884520+1 records out
452874563 bytes (453 MB, 432 MiB) copied, 4.40603 s, 103 MB/s

After:

perf stat -e branches,branch-misses tools/ldb --value_hex scan --db=test_db | dd of=/dev/null
 Performance counter stats for 'tools/ldb --value_hex scan --db=test_db':
       275,719,553      branches:u
           797,887      branch-misses:u           #    0.29% of all branches
       1.922297831 seconds time elapsed
       0.346486000 seconds user
       0.785366000 seconds sys
884520+1 records in
884520+1 records out
452874563 bytes (453 MB, 432 MiB) copied, 1.97187 s, 230 MB/s

Built with DEBUG_LEVEL=0. gcc 8.5.0

Also see #15072 Speed up Slice::DecodeHex and #15070