Score a word from a letter point table

A word game has a point value for each letter. A word scores the total of its letters' points.

Write word_score(word, points) that returns the score of word.

Input and output:

• word: a string of up to 100,000 lowercase letters a to z. It may be empty.
• points: a dictionary that maps single lowercase letters to integers between 0 and 100. It may be empty, and it may leave letters out.
• Returns an integer.

Rules:

• Every occurrence of a letter counts, so a letter that appears 3 times scores 3 times.
• A letter missing from points scores 0.
• An empty word scores 0.