Initial support for encryption on the web

[simolus3]

Add support for using sqlite3mc with web builds. This appears to be working, but there are some remaining issues to address:

• This replaces the build scripts to only compile sqlite3mc. We should compile both upstream sqlite3 and with sqlite3mc.
• Due to the way we set up virtual file systems (after loading sqlite3, not in sqlite3_os_init), we need to disable SQLITE_AUTOINIT with sqlite3mc because that extension sets up an extrainit to turn the default vfs into an encrypted vfs. That fails, as there is no default VFS at that point. The current setup is backwards-compatible (since old wasm bundles have autoinit and we can start calling sqlite3_initialize in new versions). Still, I prefer to keep AUTOINIT enabled for the default build.
• This also needs more tests.