use variable length slice for Solution to permit regtest

[LarryRuane]

This fixes the breaking of regtest caused by #505 (Change hash representation from slice to 32-byte array). There is no functional mainnet difference.

The Solution field within the block header has different sizes for regtest (36 bytes) and mainnet and testnet (1344 bytes), and #505 added the assumption that Solution is always 1344 bytes long. This is the one field whose length is compact-length encoded in the serialization, so it makes sense to treat it as variable length -- it could change in the future. (The other fields in the header are fixed-sized.) There is some unit test code that assumes 1344 bytes, but that's okay.

This PR replaces #533, which has been in draft status for a long time and seems to have been abandoned.

[LarryRuane]

To test this using regtest:

$ rm -rf /tmp/zrtest
$ mkdir /tmp/zrtest
cat <<EOF >/tmp/zrtest/zcash.conf
regtest=1
lightwalletd=1
experimentalfeatures=1
debug=1
i-am-aware-zcashd-will-be-replaced-by-zebrad-and-zallet-in-2025=1
rpcpassword=light
EOF
$ zcashd -datadir=/tmp/zrtest

In another window, at the top level of the lightwalletd source directory:

$ go run main.go --zcash-conf-path /tmp/zrtest/zcash.conf --no-tls-very-insecure --log-file /dev/stdout

Without this PR, you'll see this in the log messages:

getblock 0 failed, will retry: error parsing block: parsing block header: solution length is not 1344 as expected

[pacu]

utACK. Looks good to me.