upgrade amalgamation code

[mattn]

1. evert the data layout for an internal-use-only SQLite data structure. Applications that use SQLite should never reference internal SQLite data structures, but some do anyhow, and a change to one such data structure in 3.30.0 broke a popular and widely-deployed application. Reverting that change in SQLite, at least temporarily, gives developers of misbehaving applications time to fix their code.
2. Fix a typos in the sqlite3ext.h header file that prevented the sqlite3_stmt_isexplain() and sqlite3_value_frombind() interfaces from being called from run-time loadable extensions.

https://www.sqlite.org/changes.html

Fixes #776

[coveralls]

Coverage remained the same at 51.497% when pulling 34cdff5 on sqlite-amalgamation-3310100 into ed406dd on master.