[lazy] Use switch instead of indirect function calls.

Use a switch statement to select the search function instead of an
indirect function call. This results in a sizable performance win.

This PR is a modification of the approach taken in PR #2828.
When I measured performance for that commit, it was neutral.
However, I now see a performance regression on gcc, but still
neutral on clang. I'm measuring on the same platform, but with
newer compilers. The new approach beats both the current dev
branch and the baseline before PR #2828 was merged.

This PR is necessary for Issue #3275, to update zstd in the kernel.
Without this PR there is a large regression in greedy - btlazy2
compression speed. With this PR it is about neutral.

gcc version: 12.2.0
clang version: 14.0.6
dataset: silesia.tar

| Compiler | Level | Dev Speed (MB/s) | PR Speed (MB/s) | Delta  |
|----------|-------|------------------|-----------------|--------|
| gcc      |     5 |            102.6 |           113.7 | +10.8% |
| gcc      |     7 |             66.6 |            74.8 | +12.3% |
| gcc      |     9 |             51.5 |            58.9 | +14.3% |
| gcc      |    13 |             14.3 |            14.3 |  +0.0% |
| clang    |     5 |            108.1 |           114.8 |  +6.2% |
| clang    |     7 |             68.5 |            72.3 |  +5.5% |
| clang    |     9 |             53.2 |            56.2 |  +5.6% |
| clang    |    13 |             14.3 |            14.7 |  +2.8% |

The binary size stays just about the same for clang and gcc, measured
using the `size` command:

| Compiler | Branch | Text    | Data | BSS | Total   |
|----------|--------|---------|------|-----|---------|
| gcc      | dev    | 1127950 | 3312 | 280 | 1131542 |
| gcc      | PR     | 1123422 | 2512 | 280 | 1126214 |
| clang    | dev    | 1046254 | 3256 | 216 | 1049726 |
| clang    | PR     | 1048198 | 2296 | 216 | 1050710 |