[foreign-memaccess+abi] RFR: 8314592: Add shortcut to SymbolLookup::find [v2]

[Maurizio Cimadamore]

> This patch adds a default method in `SymbolLookup`, namely `SymbolLookup::get`. Its implementation is very simple: it calls `SymbolLookup::find` and then `Optional::get` on the result. This allows to simplify many clients that end up calling `Optional::get` or `Optional::orElseThrow` after a symbol lookup, but still retains the compositional advantage of the optional-returning lookup primitive.

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Address review comments

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Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/871/files
  - new: https://git.openjdk.org/panama-foreign/pull/871/files/ba015090..2409a7ef

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=871&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=871&range=00-01

  Stats: 8 lines in 1 file changed: 7 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/871.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/871/head:pull/871

PR: https://git.openjdk.org/panama-foreign/pull/871