silentpayments: tweak spend pubkey using ecmult_gen_var (~25% scanning speedup for small N) #1914

Having this available as a global constant allows to introduce an
alternative `ecmult_gen_gej` function that doesn't need access to
a context, see next commit.

Note that the precomputed constant takes the name of the function that
previously generated it at run-time (`secp256k1_ecmult_gen_scalar_diff`),
while the function is now renamed to include the "compute" verb
(`secp256k1_ecmult_gen_compute_scalar_diff`), to match the naming
of the table generation function.

Can be reviewed with `--color-moved=dimmed-zebra` for easier
checking of the move-only parts.

Add faster variable-time variants for generator point multiplication.
This is essentially `ecmult_gen` without side-channel mitigations and
without requiring a context object. Intended for use cases where the
scalar is not representing sensitive data.

On my arm64 machine, this is ~86% faster than the constant-time variant
(with the default build table size, i.e. ECMULT_GEN_KB=86):

```
$ ./build/bin/bench_ecmult
Benchmark                     ,    Min(us)    ,    Avg(us)    ,    Max(us)

ecmult_gen                    ,     9.32      ,     9.35      ,     9.60
ecmult_gen_var                ,     5.02      ,     5.02      ,     5.02
.....
```

Note that the tweak t_k is not considered a long-term secret, so using
variable-time functions for calculating P_k = B_spend + t_k * G seems fine.
This leads to a ~25% scanning speedup if the number of outputs is small (N <= 10):

master:
```
$ ./build/bin/bench silentpayments_scan_nomatch
Benchmark                               ,    Min(us)       ,    Avg(us)       ,    Max(us)

silentpayments_scan_nomatch_N=2         ,       39.3       ,       39.3       ,       39.3
silentpayments_scan_nomatch_N=5         ,       40.4       ,       40.4       ,       40.4
silentpayments_scan_nomatch_N=10        ,       43.3       ,       43.3       ,       43.3
silentpayments_scan_nomatch_N=100       ,       89.0       ,       89.6       ,       91.0
silentpayments_scan_nomatch_N=1000      ,      560.0       ,      563.0       ,      568.0
silentpayments_scan_nomatch_N=2323      ,     1254.0       ,     1259.0       ,     1271.0
silentpayments_scan_nomatch_N=23250     ,    12196.0       ,    12209.0       ,    12228.0
```

PR branch (using ecmult_gen_var for calculating t_k * G):

```
$ ./build/bin/bench silentpayments_scan_nomatch
Benchmark                               ,    Min(us)       ,    Avg(us)       ,    Max(us)

silentpayments_scan_nomatch_N=2         ,       30.5       ,       30.5       ,       30.5
silentpayments_scan_nomatch_N=5         ,       31.7       ,       31.7       ,       31.7
silentpayments_scan_nomatch_N=10        ,       34.6       ,       34.6       ,       34.6
silentpayments_scan_nomatch_N=100       ,       80.0       ,       81.2       ,       85.0
silentpayments_scan_nomatch_N=1000      ,      550.0       ,      552.0       ,      554.0
silentpayments_scan_nomatch_N=2323      ,     1239.0       ,     1244.0       ,     1253.0
silentpayments_scan_nomatch_N=23250     ,    12164.0       ,    12182.0       ,    12214.0
```

Speedups:

N=2: ~28.85%
N=5: ~27.44%
N=10: ~25.14%
N=100: ~10.34%
N=1000: ~1.99%